U.S. patent application number 11/495693 was filed with the patent office on 2006-11-30 for rfid-tag communication device.
This patent application is currently assigned to Brother Kogyo Kabushiki Kaisha. Invention is credited to Yuji Kiyohara, Takuya Nagai, Kazunari Taki.
Application Number | 20060267734 11/495693 |
Document ID | / |
Family ID | 34840140 |
Filed Date | 2006-11-30 |
United States Patent
Application |
20060267734 |
Kind Code |
A1 |
Taki; Kazunari ; et
al. |
November 30, 2006 |
RFID-tag communication device
Abstract
An RFID-tag communication device including (a) an antenna from
which a transmission signal is transmitted toward an RFID tag and
through which a reply signal transmitted from the RFID tag in
response to the transmission signal is received, for radio
communication with the RFID tag, (b) a first-cancel-signal output
portion operable to generate a digital first cancel signal for
suppressing a leakagel signal from a received signal which is
received through the antenna and which contains the leakage signal
as well as the rely signal, the leakage signal being a part of the
transmission signal which is transmitted from the antenna and which
is returned to and received by the antenna, (c) a
first-cancel-signal control portion operable to control an
amplitude and/or a phase of the first cancel signal generated by
the first-cancel-signal output portion, (d) a first-cancel-signal
D/A converting portion operable to convert the first cancel signal
generated by the first-cancel-signal output portion, into an analog
signal, (e) a first signal combining portion operable to combine
together the analog first cancel signal generated by the
first-cancel-signal D/A converting portion, and the received
signal, to obtain a first composite signal.
Inventors: |
Taki; Kazunari; (Nagoya-shi,
JP) ; Kiyohara; Yuji; (Nagoya-shi, JP) ;
Nagai; Takuya; (Nagoya-shi, JP) |
Correspondence
Address: |
BAKER BOTTS LLP;C/O INTELLECTUAL PROPERTY DEPARTMENT
THE WARNER, SUITE 1300
1299 PENNSYLVANIA AVE, NW
WASHINGTON
DC
20004-2400
US
|
Assignee: |
Brother Kogyo Kabushiki
Kaisha
Nagoya-shi
JP
|
Family ID: |
34840140 |
Appl. No.: |
11/495693 |
Filed: |
July 31, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP05/01620 |
Feb 3, 2005 |
|
|
|
11495693 |
Jul 31, 2006 |
|
|
|
Current U.S.
Class: |
340/10.4 ;
340/572.4 |
Current CPC
Class: |
G06K 19/0723 20130101;
G06K 7/10039 20130101; G06K 7/0008 20130101 |
Class at
Publication: |
340/010.4 ;
340/572.4 |
International
Class: |
H04Q 5/22 20060101
H04Q005/22 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 4, 2004 |
JP |
2004-028013 |
Aug 31, 2004 |
JP |
2004-251854 |
Claims
1. An RFID-tag communication device including an antenna from which
a transmission signal is transmitted toward an RFID tag and through
which a reply signal transmitted from the RFID tag in response to
the transmission signal is received, for radio communication with
the RFID tag, said RFID-tag communication device comprising: a
first-cancel-signal output portion operable to generate a digital
first cancel signal for suppressing a leakagel signal from a
received signal which is received through said antenna and which
contains said leakage signal as well as said rely signal, said
leakage signal being a part of said transmission signal which is
transmitted from said antenna and which is returned to and received
by said antenna; a first-cancel-signal control portion operable to
control an amplitude and/or a phase of said first cancel signal
generated by said first-cancel-signal output portion; a
first-cancel-signal D/A converting portion operable to convert said
first cancel signal generated by said first-cancel-signal output
portion, into an analog signal; and a first signal combining
portion operable to combine together the analog first cancel signal
generated by said first-cancel-signal D/A converting portion, and
said received signal, to obtain a first composite signal.
2. The RFID-tag communication device according to claim 1, further
comprising: a second-cancel-signal output portion operable to
generate a digital second cancel signal for suppressing said
leakage signal from said received signal; a second-cancel-signal
control portion operable to control an amplitude and/or a phase of
said second cancel signal generated by said second-cancel-signal
output portion; a second-cancel-signal D/A converting portion
operable to convert said second cancel signal generated by said
second-cancel-signal output portion, into an analog signal; and a
second signal combining portion operable to combine together the
analog second cancel signal generated by said second-cancel-signal
D/A converting portion, and said received signal, to obtain a
second composite signal.
3. The RFID-tag communication device according to claim 2, wherein
said first and second cancel signals have respective different
frequencies.
4. The RFID-tag communication device according to claim 2, further
comprising an amplifying portion interposed between said first and
second signal combining portions and operable to amplify an
amplitude of said first composite signal generated by said first
signal combining portion.
5. The RFID-tag communication device according to claim 2, further
comprising an amplifying portion operable to amplify an amplitude
of said second composite signal generated by said second signal
combining portion.
6. The RFID-tag communication device according to claim 1, further
comprising a received-signal-amplitude detecting portion operable
to detect an amplitude of said received signal, and said
first-cancel-signal control portion controls an amplitude of said
first cancel signal, on the basis of the amplitude of said received
signal detected by said received-signal-amplitude detecting
portion.
7. The RFID-tag communication device according to claim 1, further
comprising a first-composite-signal-amplitude detecting portion
operable to detect an amplitude of said first composite signal
generated by said first signal combining portion, and said
first-cancel-signal control portion controls a phase of said-first
cancel signal, on the basis of the amplitude of said first
composite signal detected by said first-composite-signal-amplitude
detecting portion.
8. The RFID-tag communication device according to claim 2, further
comprising a first-composite-signal-amplitude detecting portion
operable to detect an amplitude of said first composite signal
generated by said first signal combining portion, and wherein said
first-cancel-signal control portion controls a phase of said first
cancel signal, on the basis of the amplitude of said first
composite signal detected by said first-composite-signal-amplitude
detecting portion, and said second-cancel-signal control portion
controls an amplitude of said second cancel signal, on the basis of
the amplitude of said first composite signal detected by said
first-composite-signal-amplitude detecting portion.
9. The RFID-tag communication device according to claim 2, further
comprising a demodulating portion operable to demodulate said
second composite signal generated by said second signal combining
portion, and a direct-current-component detecting portion operable
to detect a direct current component of the second composite signal
demodulated by said demodulating portion, and wherein said
second-cancel-signal control portion controls a phase of said
second cancel signal on the basis of the direct current component
of the demodulated second composite signal detected by said
direct-current-component detecting portion.
10. The RFID-tag communication device according to claim 9, further
comprising: a first-composite-signal-amplitude detecting portion
operable to detect an amplitude of said first composite signal
generated by said first signal combining portion; a
digital-transmission-signal output portion operable to generate
said transmission signal in the form of a digital signal; a
transmission-signal D/A converting portion operable to covert the
digital transmission signal generated by said
digital-transmission-signal output portion, into an analog signal;
a first-composite-signal A/D converting portion interposed between
said first signal combining portion and said
first-composite-signal-amplitude detecting portion and operable to
convert said first composite signal generated by said first signal
combining portion, into a digital signal; a second-composite-signal
A/D converting portion interposed between said second signal
combining portion and said demodulating portion and operable to
convert said second composite signal generated by said second
signal combining portion, into an analog signal; and a
received-signal A/D converting portion operable to convert said
received signal into an analog signal, wherein said
first-cancel-signal control portion controls a phase of said first
cancel signal, on the basis of the amplitude of said first
composite signal detected by said first-composite-signal-amplitude
detecting portion, and wherein said first-cancel-signal D/A
converting portion, said second-cancel-signal D/A converting
portion, said transmission-signal D/A converting portion, said
first-composite-signal A/D converting portion, said
second-composite-signal A/D converting portion and said
receive-signal A/D converting portion use a common clock
signal.
11. The RFID-tag communication device according to claim 1, further
comprising: a local-oscillation-signal output portion operable to
generate a local oscillation signal: a first up-converter operable
to increase a frequency of the analog transmission signal generated
by said transmission-signal D/A converting portion, by an amount
corresponding to a frequency of said local oscillation signal
generated by said local-oscillation-signal output portion; and a
first down-converter operable to reduce a frequency of said first
composite signal generated by said first signal combining portion,
by the frequency of said local oscillation signal generated by said
local-oscillation-signal output portion.
12. The RFID-tag communication device according to claim 11,
further comprising a second down-converter operable to reduce a
frequency of said received signal, by an amount corresponding to
the frequency of said local oscillation signal generated by said
local-oscillation-signal output portion.
13. The RFID-tag communication device according to claim 1, further
comprising a digital-transmission-signal output portion operable to
generate said transmission signal in the form of a digital signal,
and a table storing a sine-wave or cosine-wave table which
represents predetermined sampling values corresponding to
respective phases at predetermined sampling points, wherein said
digital-transmission-signal output portion generates said
transmission signal on the basis of said sine-wave or cosine-wave
table.
14. The RFID-tag communication device according to claim 13,
wherein said first-cancel-signal output portion generates said
first cancel signal on the basis of said sine-wave or cosine-wave
table, and said first-cancel-signal control portion controls a
phase of said first cancel signal by changing positions of said
sine-wave or cosine-wave table from which said sampling values are
read out.
15. The RFID-tag communication device according to claim 14,
wherein said first-cancel-signal control portion controls an
amplitude of said first cancel signal by multiplying the digital
signal generated on the basis of said sine-wave or cosine-wave
table, by a predetermined control value.
16. The RFID-tag communication device according to claim 13,
wherein said second-cancel-signal output portion generates said
second cancel signal on the basis of said sine-wave or cosine-wave
table, and said second-cancel-signal control portion controls a
phase of said second cancel signal, by changing positions of said
sine-wave or cosine-wave table from which said sampling values are
read out.
17. The RFID-tag communication device according to claim 16,
wherein said second-cancel-signal control portion controls an
amplitude of said second cancel signal, by multiplying the digital
signal generated on the basis of said sine-wave or cosine-wave
table, by a predetermined control value.
18. The RFID-tag communication device according to claim 13,
further comprising a local-oscillation-signal output portion
operable to generate a local oscillation signal, and a
down-converter operable to reduce an amplitude of said received
signal by an amount corresponding to a frequency of said local
oscillation signal generated by said local-oscillation-signal
output portion, and wherein said first-cancel-signal control
portion controls an amplitude and a phase of said first cancel
signal on the basis of said received signal or an output of said
down-converter, and controls the phase of said first cancel signal
on the basis of said second composite signal generated by said
second signal combining portion.
19. The RFID-tag communication device according to claim 1, further
comprising: a third-cancel-signal output portion operable to
generate a third cancel signal for suppressing said leakage signal
from said received signal; a third signal combining portion
operable to combine together said third cancel signal generated by
said third-cancel-signal output portion and said received signal,
to obtain a third composite signal, and wherein said
first-cancel-signal control portion controls an amplitude of said
first cancel signal on the basis of said third composite signal
generated by said third signal combining portion.
20. The RFID-tag communication device according to claim 11,
wherein said local-oscillation-signal output portion is operable to
effect hopping of frequency of said local oscillation signal.
21. The RFID-tag communication device according to claim 1, further
comprising: a sine-wave-signal generating portion operable to
generate a first sine-wave signal and a second sine-wave signal
which have respective different phases; an amplitude control
portion operable to control amplitudes of said first and second
sine-wave signals generated by said sine-wave-signal generating
portion; and a sine-wave synthesizing portion operable to combine
together said first and second sine-wave signals the amplitudes of
which have been controlled by said amplitude control portion, to
synthesize a composite sine-wave signal for radio
communication.
22. The RFID-tag communication device according to claim 21,
wherein said sine-wave-signal generating portion generates said
first and second sine-wave signals which have a phase difference of
about 90.degree., and said sine-wave synthesizing portion generates
said composite sine-wave signal having an amplitude and a phase
which are different from those of said first and second sine-wave
signals.
23. The RFID-tag communication device according to claim 22,
wherein said sine-wave-signal generating portion includes a
carrier-wave output portion operable to generate said first
sine-wave signal as said transmission signal in the form of a
carrier wave for obtaining an access to said RFID tag, said antenna
comprising a transmitter portion operable to transmit said carrier
component generated by said carrier-wave output portion, toward
said RFID tag, and a receiver portion operable to receive said
reply signal transmitted from said RFID tag in response to said
carrier wave, said sine-wave synthesizing portion generating said
composite sine-wave signal as said first cancel signal which has an
amplitude substantially equal to an amplitude of said leakage
signal, and a phase that is reversed with respect to that of said
leakage signal.
24. The RFID-tag communication device according to claim 23,
wherein said sine-wave generating portion includes a first
digital-to-analog converting portion operable to convert a set of
sine-wave sampling values into said first sine-wave signal, and a
second digital-to-analog converting portion operable to convert a
set of sine-wave sampling values into said second sine-wave
signal.
25. The RFID-tag communication device according to claim 23,
wherein said first signal combining portion synthesizes said first
composite signal by combining together said received signal
received by said receiver portion, and said first cancel signal
generated by said sine-wave synthesizing portion, said RFID-tag
communication device further comprising an analog-to-digital
converting portion operable to convert said first composite signal
into an analog signal, and wherein said first digital-to-analog
converting portion and said analog-to-digital converting portion
receive a common clock signal.
26. The RFID-tag communication device according to claim 25,
further comprising an up-converter operable to increase a frequency
of said composite sine-wave signal generated by said sine-wave
synthesizing portion, and a down-converter operable to reduce a
frequency of said first composite signal generated by said first
signal combining portion.
27. The RFID-tag communication device according to claim 25,
wherein said sine-wave synthesizing portion synthesizes, in
addition to said first cancel signal, a second cancel signal having
an amplitude substantially equal to an amplitude of a carrier
component of said first composite signal, and a phase that is
reversed with respect to that of said carrier component of said
first composite signal.
28. The RFID-tag communication device according to claim 21,
wherein said antenna has a plurality of antenna elements for radio
communication with said RFID tag, and said sine-wave synthesizing
portion generates said composite sine-wave signal a phase of which
has been controlled with respect to said plurality of antenna
elements, such that the generated composite sine-wave signal is
transmitted toward said RFID tag while a directivity of said
plurality of antenna elements is controlled.
29. The RFID-tag communication device according to claim 28,
wherein said sine-wave synthesizing portion transmits, toward said
RFID tag, said composite sine-wave signal at least the phase of
which has been controlled, such that the direction in which the
plurality of antennas have a highest gain and which is temporarily
held is sequentially changed
30. The RFID-tag communication device according to claim 28,
wherein said sine-wave synthesizing portion transmits, toward said
RFID tag, said composite sine-wave signal an amplitude and the
phase of which have been controlled, such that a sensitivity of
reception by said plurality of antennas of said reply signal
transmitted from said RFID tag is maximized.
31. The RFID-tag communication device according to claim 28,
wherein said sine-wave generating portion includes a first
digital-analog converter operable to convert a set of sine-wave
sampling values into said first sine-wave signal, and a second
digital-analog converter operable to convert a set of sine-wave
sampling values into said second sine-wave signal.
32. The RFID-tag communication device according to claim 28,
further comprising an up-converter operable to increase a frequency
of said composite sine-wave signal synthesized by said sine-wave
synthesizing portion.
33. The RFID-tag communication device according to claim 21,
wherein said amplitude control portion includes a first variable
attenuator operable to control the amplitude of said first
sine-wave signal, and a second variable attenuator operable to
control the amplitude of said second sine-wave signal, and a
polarity switching portion operable to change polarities of the
first and second sine-wave signals.
34. The RFID-tag communication device according to claim 33,
wherein said amplitude control portion includes a logic circuit
operable to generate a control signal in the form of a serial
signal, and a registering portion operable to convert said serial
signal into a parallel signal and generate an amplitude control
signal and a polarity control signal on the basis of said parallel
signal, said amplitude control signal and said polarity control
signal being applied to said first and second variable attenuators
and said polarity switching portion for controlling the amplitudes
and polarities of said first and second sine-wave signals.
Description
[0001] The present application is a Continuation-in-Part of
International Application No. PCT/JP2005/001620 filed Feb. 3, 2005,
which claims the benefits of Japanese Patent Application No.
2004-028013 filed Feb. 4, 2004 and No. 2004-251854 filed Aug. 21,
2004.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an improvement of a
radio-frequency identification tag (RFID-tag) communication device
capable of radio communication with radio-frequency identification
(RFID) tags for writing and reading information on and from the
RFID tags.
[0004] 2. Description of the Related Art
[0005] There is known an RFID system (radio-frequency
identification system) wherein an RFID-tag (radio-frequency
identification tag) communication device (e.g., an interrogator)
reads out information, in a non-contact fashion, from small RFID
tags (radio-frequency identification tags, e.g., transponders) on
which information is written. In this RFID system, the RFID-tag
communication device is capable of reading out the information from
the RFID tags, even where the RFID tags are contaminated or located
at positions invisible from the radio-frequency tag communication
device. For this reason, the RFID system is expected to be used in
various fields, such as management and inspection of articles of
commodity.
[0006] Generally, the RFID-tag communication device is arranged to
effect radio communication with the RFID tags, by transmitting a
suitable transmission signal from its antenna toward the RFID tags,
and receiving through the antenna a reply signal which is
transmitted from the RFID tags in response to the signal received
from the RFID-tag communication device. A leakage signal that is a
part of the transmission signal which is transmitted from the
RFID-tag communication device toward the RFID tags and returned to
and received by the communication device and which has a relatively
high intensity may be mixed with the reply signal transmitted from
the RFID tags, so that the intensity of the signals received by the
RFID-tag communication device may exceed a permissible upper limit
of an amplifier provided in the RFID-tag communication device,
whereby the received signals cannot be sufficiently amplified by
the amplifier, and the reply signal component cannot be
sufficiently amplified. Thus, the known RFID-tag communication
device suffers from a risk of insufficiency of the signal-to-noise
ratio. In view of this problem, there have been proposed techniques
for suppressing the leakage signal. JP-8-122429 A discloses an
example of an interference compensating device used in a mobile
identification system.
[0007] However, the conventional technique to suppress the leakage
signal due to the transmission signal transmitted from the RFID-tag
communication device and returned to the communication device uses
primarily analog processing to control the amplitude and phase of a
compensating signal (canceling signal) to suppress the leakage
signal, and therefore requires a comparatively expensive large
phase shifter. The conventional technique has a further problem of
difficulty of control of the phase shifter. Thus, there has not
been a simple RFID-tag communication device developed for
suppression of the leakage signal.
SUMMARY OF THE INVENTION
[0008] The present invention was made in view of the background art
described above. It is therefore an object of this invention to
provide an RFID-tag communication device which is simple in
construction and which is capable of suppressing a leakage signal
that is a part of the transmission signal which is transmitted from
the communication device and which is returned to and received by
the communication device.
[0009] The object indicated above may be achieved according to the
principle of this invention, which provides an RFID-tag
communication device including an antenna from which a transmission
signal is transmitted toward an RFID tag and by which a reply
signal transmitted from the RFID tag in response to the
transmission signal is received, for radio communication with the
RFID tag, the RFID-tag communication device comprising: a
first-cancel-signal output portion operable to generate a digital
first cancel signal for suppressing a leakage signal from a
received signal which is received through the antenna and which
contains the leakage signal as well as the rely signal, the leakage
signal being a part of the transmission signal which is transmitted
from the antenna and which is returned to and received by the
antenna; a first-cancel-signal control portion operable to control
an amplitude and/or a phase of the first cancel signal generated by
the first-cancel-signal output portion; a first-cancel-signal D/A
converting portion operable to convert the first cancel signal
generated by the first-cancel-signal output portion, into an analog
signal; and a first signal combining portion operable to combine
together the analog first cancel signal generated by the
first-cancel-signal D/A converting portion, and the received
signal, to obtain a first composite signal.
[0010] The RFID-tag communication device of the present invention,
which includes the first-cancel-signal output portion,
first-cancel-signal control portion, first-cancel-signal D/A
converting portion and first signal combining portion, as described
above, does not require a phase shifter for controlling the first
cancel signal, and permits easy control of the amplitude and/or
phase of the first cancel signal by digital processing. Namely, the
present RFID-tag communication device is simple in construction but
is capable of suppressing the leakage signal which is a part of the
transmission signal which is transmitted from the antenna and which
is returned to and received by the antenna.
[0011] According to a first preferred form of the present
invention, the RFID-tag communication device further comprises: a
second-cancel-signal output portion operable to generate a digital
second cancel signal for suppressing the leakage signal from the
received signal; a second-cancel-signal control portion operable to
control an amplitude and/or a phase of the second cancel signal
generated by the second-cancel-signal output portion; a
second-cancel-signal D/A converting portion operable to convert the
second cancel signal generated by the second-cancel-signal output
portion, into an analog signal; and a second signal combining
portion operable to combine together the analog second cancel
signal generated by the second-cancel-signal D/A converting
portion, and the received signal, to obtain a second composite
signal. The RFID-tag communication device according to the present
first preferred form of the invention permits secondary suppression
of the leakage signal at the second signal combining portion, as
well as primary suppression of the leakage signal at the first
signal combining portion, making it possible to further improve the
signal-to-noise ratio.
[0012] In a first advantageous arrangement of the first preferred
form of the invention, the first and second cancel signals have
respective different frequencies. Therefore, the first and second
cancel signals can be easily controlled according to the control
signals corresponding to the frequencies of the first and second
cancel signals.
[0013] In a second advantageous arrangement of the first preferred
form of the invention, the RFID-tag communication device further
comprises an amplifying portion interposed between the first and
second signal combining portions and operable to amplify an
amplitude of the first composite signal generated by the first
signal combining portion. In this arrangement, the first composite
signal and the second cancel signal can be suitably combined
together by the second signal combining portion, to obtain the
second composite signal. Further, the amplification of the first
composite signal by the amplifying portion permits reception of the
reply signal with high sensitivity.
[0014] In a third advantageous arrangement of the first preferred
form of the invention, the RFID-tag communication device further
comprises an amplifying portion operable to amplify an amplitude of
the second composite signal generated by the second signal
combining portion. This arrangement permits detection of the second
composite signal with high sensitivity, by analog-to-digital
conversion of the second composite signal or demodulation of the
second composite signal.
[0015] According to a second preferred form of the present
invention, the RFID-tag communication device further comprises a
received-signal-amplitude detecting portion operable to detect an
amplitude of the received signal, and the first-cancel-signal
control portion controls an amplitude of the first cancel signal,
on the basis of the amplitude of the received signal detected by
the received-signal-amplitude detecting portion. The present form
of the invention permits effective suppression of the leakage
signal contained in the received signal.
[0016] According to a third preferred form of the present
invention, the RFID-tag communication device according to claim 1,
further comprising a first-composite-signal-amplitude detecting
portion operable to detect an amplitude of the first composite
signal generated by the first signal combining portion, and the
first-cancel-signal control portion controls a phase of the first
cancel signal, on the basis of the amplitude of the first composite
signal detected by the first-composite-signal-amplitude detecting
portion. The present form of the invention permits effective
suppression of the leakage signal contained in the received
signal.
[0017] In a fourth advantageous arrangement of the first preferred
forms of the present invention, the RFID-tag communication device
further comprises a first-composite-signal-amplitude detecting
portion operable to detect an amplitude of the first composite
signal generated by the first signal combining portion. In this
communication device, the first-cancel-signal control portion
controls a phase of the first cancel signal, on the basis of the
amplitude of the first composite signal detected by the
first-composite-signal-amplitude detecting portion, and the
second-cancel-signal control portion controls an amplitude of the
second cancel signal, on the basis of the amplitude of the first
composite signal detected by the first-composite-signal-amplitude
detecting portion. This arrangement permits effective suppression
of the leakage signal contained in the received signal.
[0018] In a fifth advantageous arrangement of the first preferred
form of the invention, the RFID-tag communication device further
comprises a demodulating portion operable to demodulate the second
composite signal generated by the second signal combining portion,
and a direct-current-component detecting portion operable to detect
a direct current component of the second composite signal
demodulated by the demodulating portion. In this communication
device, the second-cancel-signal control portion controls a phase
of the second cancel signal on the basis of the direct current
component of the demodulated second composite signal detected by
the direct-current-component detecting portion. This arrangement
permits effective suppression of the leakage signal contained in
the received signal.
[0019] Preferably, the RFID-tag communication device according to
the fifth advantageous arrangement of the first preferred form of
the invention further comprises: a first-composite-signal-amplitude
detecting portion operable to detect an amplitude of the first
composite signal generated by the first signal combining portion; a
digital-transmission-signal output portion operable to generate the
transmission signal in the form of a digital signal; a
transmission-signal D/A converting portion operable to covert the
digital transmission signal generated by the
digital-transmission-signal output portion, into an analog signal;
a first-composite-signal A/D converting portion interposed between
the first signal combining portion and the
first-composite-signal-amplitude detecting portion and operable to
convert the first composite signal generated by the first signal
combining portion, into a digital signal; a second-composite-signal
A/D converting portion interposed between the second signal
combining portion and the demodulating portion and operable to
convert the second composite signal generated by the second signal
combining portion, into an analog signal; and a received-signal A/D
converting portion operable to convert the received signal into an
analog signal, wherein the first-cancel-signal control portion
controls a phase of the first cancel signal, on the basis of the
amplitude of the first composite signal detected by the
first-composite-signal-amplitude detecting portion. In this
communication device, the first-cancel-signal D/A converting
portion, the second-cancel-signal D/A converting portion, the
transmission-signal D/A converting portion, the
first-composite-signal A/D converting portion, the
second-composite-signal A/D converting portion and the
receive-signal A/D converting portion use a common clock signal.
The present communication device does not suffer from a difference
in the reference phase between the transmission signal and the
received signal, and permits effective suppression of the leakage
signal contained in the received signal. In this respect, it is
noted that the demodulation of the received signal the frequency of
which has been reduced to an intermediate frequency has a risk of
considerable generation of a relatively low frequency component
upon the demodulation due to a difference between the frequency of
the clock signal of the A/D converting portions and the
intermediate frequency. However, this risk can be prevented by
using the common clock signal for the digital-to-analog conversion
and the analog-to-digital conversion.
[0020] According to a fourth preferred form of the present
invention, the RFID-tag communication device further comprises: a
local-oscillation-signal output portion operable to generate a
local oscillation signal: a first up-converter operable to increase
a frequency of the analog transmission signal generated by the
transmission-signal D/A converting portion, by an amount
corresponding to a frequency of the local oscillation signal
generated by the local-oscillation-signal output portion; and a
first down-converter operable to reduce a frequency of the first
composite signal generated by the first signal combining portion,
by the frequency of said local oscillation signal generated by the
local-oscillation-signal output portion. In the present
communication device, the analog-to-digital conversion of the first
composite signal and the digital-to-analog conversion of the
transmission signal can be effected with a simple converter
arrangement using relatively inexpensive A/D and D/A
converters.
[0021] In a first advantageous arrangement of the fourth preferred
form of this invention, the RFID-tag communication device further
comprises a second down-converter operable to reduce a frequency of
the received signal, by an amount corresponding to the frequency of
the local oscillation signal generated by the
local-oscillation-signal output portion. In this communication
device, the analog-to-digital conversion of the received signal can
be effected by using a simple converter arrangement using a
relatively inexpensive A/D converter.
[0022] According to a sixth advantageous arrangement of the first
preferred form of this invention, the RFID-tag communication device
further comprises a digital-transmission-signal output portion
operable to generate the transmission signal in the form of a
digital signal, and a table storing a sine-wave or cosine-wave
table which represent predetermined sampling values corresponding
to respective phases at predetermined sampling points, wherein the
digital-transmission-signal output portion generates the
transmission signal on the basis of the sine-wave or cosine-wave
table. In this communication device, the
digital-transmission-signal output portion can generate the digital
transmission signal, with a relatively simple arrangement.
[0023] In the sixth advantageous arrangement of the first preferred
form of the invention, the first-cancel-signal output portion is
preferably arranged to generate the first cancel signal on the
basis of the sine-wave or cosine-wave table, and the
first-cancel-signal control portion is preferably arranged to
control a phase of the first cancel signal by changing positions of
the sine-wave or cosine-wave table from which the sampling values
are read out. In this communication device, the first-cancel-signal
output portion can generate the digital first cancel signal with a
relatively simple arrangement, and the phase of the first cancel
signal can be easily controlled.
[0024] Further, the invention, the first-cancel-signal control
portion is preferably arranged to control an amplitude of the first
cancel signal by multiplying the digital signal generated on the
basis of the sine-wave or cosine-wave table, by a predetermined
control value. In this communication device, the amplitude of the
first cancel signal can be easily controlled.
[0025] In the sixth advantageous arrangement of the first preferred
form of the invention, the second-cancel-signal output portion is
preferably arranged to generate the second cancel signal on the
basis of the sine-wave or cosine-wave table, and the
second-cancel-signal control portion is preferably arranged to
control a phase of the second cancel signal, by changing positions
of the sine-wave or cosine-wave table from which the sampling
values are read out. In this communication device, the
second-cancel-signal output portion can generate the digital second
cancel signal with a relatively simple arrangement, and the phase
of the second cancel signal can be easily controlled.
[0026] Further, the second-cancel-signal control portion is
preferably arranged to control an amplitude of the second cancel
signal, by multiplying the digital signal generated on the basis of
the sine-wave or cosine-wave table, by a predetermined control
value. In this communication device, the amplitude of the first
cancel signal can be easily controlled.
[0027] In the sixth advantageous arrangement of the first preferred
form of the invention, the RFID-tag communication device preferably
further comprises a local-oscillation-signal output portion
operable to generate a local oscillation signal, and a
down-converter operable to reduce an amplitude of said received
signal by an amount corresponding to a frequency of the local
oscillation signal generated by the local-oscillation-signal output
portion. In this case, the first-cancel-signal control portion
controls an amplitude and a phase of the first cancel signal on the
basis of the received signal or an output of the down-converter,
and controls the phase of the first cancel signal on the basis of
the second composite signal generated by the second signal
combining portion. This arrangement permits effective suppression
of the leakage signal contained in the received signal. Where the
first-cancel-signal control portion is arranged to set initial
values of the amplitude and phase of the first cancel signal on the
basis of the received signal or the first composite signal, the
time required for subsequent control of the phase of the first
cancel signal can be reduced.
[0028] According to a fifth preferred form of this invention, the
RFID-tag communication device further comprises: a
third-cancel-signal output portion operable to generate a third
cancel signal for supressing the leakage signal from the received
signal; a third signal combining portion operable to combine
together the third cancel signal generated by the
third-cancel-signal output portion and the received signal, to
obtain a third composite signal. In this form of the invention, the
first-cancel-signal control portion controls an amplitude of the
first cancel signal on the basis of the third composite signal
generated by the third signal combining portion. In this
communication device, the reply signal can be detected with high
sensitivity, even where the received signal contains a
comparatively large amount of the leakage signal (that is a part of
the transmission signal transmitted from and returned to the
communication device), or contains noise signals mixed therein due
to reflection of the transmitted transmission signal by any
structural body located near the communication device. In addition,
the present communication device permits the first signal combining
portion to have a relatively low upper limit of its input voltage,
thereby making it possible to reduce the amount of generation of
noises.
[0029] In a second advantageous arrangement of the fourth preferred
form of the invention, the local-oscillation-signal output portion
is operable to effect hopping of frequency of the local oscillation
signal. This arrangement is effective to prevent the
local-oscillation-signal output portion from disturbing or being
disturbed by an operation of radio communication not associated
with the radio communication with the RFID tag.
[0030] According to a sixth preferred form of the present
invention, the RFID-tag communication device further comprises: a
sine-wave-signal generating portion operable to generate a first
sine-wave signal and a second sine-wave signal which have
respective different phases; an amplitude control portion operable
to control amplitudes of the first and second sine-wave signals
generated by the sine-wave-signal generating portion; and a
sine-wave synthesizing portion operable to combine together the
first and second sine-wave signals the amplitudes of which have
been controlled by the amplitude control portion, to synthesize a
composite sine-wave signal for radio communication. The second
sine-wave signal consists of a first component the phase of which
is different by 90.degree. from that of the first sine-wave signal,
and a second component which has the same phase as that of the
second sine-wave signal. The second component is combined with the
second sine-wave signal into a sine-wave signal, while the first
component is a cosine signal. The sine-wave signal and the
cosine-wave signals which have respective different amplitudes are
combined together to synthesize a composite sine-wave signal. Thus,
the present RFID-tag communication device has a simple arrangement
for changing the phase of the composite sine-wave signal, namely, a
simple inexpensive phase shifting circuit arrangement, even where
the antenna has a relatively large number of elements.
[0031] Where the antenna includes a plurality of transmitter
elements, the sine-wave-signal generating portion is commonly used
for the plurality of transmitter elements of the antenna, and the
first and second sine-wave signals are amplified by a plurality of
amplitude control portions corresponding to the respective
transmitter elements, and are combined together by a plurality of
sine-wave synthesizing portions corresponding to the respective
transmitter elements. In this case, the circuit arrangement
associated with the generation of the first and second sine-wave
signals is simple and is accordingly inexpensive, even where the
antenna has a relatively large number of transmitter elements.
[0032] In a first advantageous arrangement of the sixth preferred
form of the invention, the sine-wave-signal generating portion
generates the first and second sine-wave signals which have a phase
difference of about 90.degree., and the sine-wave synthesizing
portion generates the composite sine-wave signal having an
amplitude and a phase which are different from those of the first
and second sine-wave signals. In this arrangement, the first and
second sine-wave signals can be provided as the sine-wave and
cosine-wave signals, which are combined together by the
sine-wave-signal combining portion to synthesize the composite
sine-wave signal, so that the amplitude of the composite sine-wave
signal can be easily controlled.
[0033] In the first advantageous arrangement of the sixth preferred
form of the invention, the sine-wave-signal generating portion
preferably includes a carrier-wave output portion operable to
generate the first sine-wave signal as the transmission signal in
the form of a carrier wave for obtaining an access to said RFID
tag. In this case, the antenna comprises a transmitter portion
operable to transmit the carrier wave generated by the carrier-wave
output portion, toward the RFID tag, and a receiver portion
operable to receive the reply signal transmitted from the RFID tag
in response to the carrier wave, and the sine-wave synthesizing
portion generates the composite sine-wave signal as said first
cancel signal which has an amplitude substantially equal to an
amplitude of the leakage signal, and a phase that is reversed with
respect to that of the leakage signal. This arrangement permits
detection of the reply signal with high sensitivity, owing to the
first cancel signal generated by the sine-wave synthesizing portion
to suppress or offset (cancel) the leakage signal that is a part of
the transmission signal or carrier wave which is transmitted from
the transmitter portion and which is returned to and received by
the receiver portion.
[0034] In the first advantageous arrangement of the sixth preferred
form of the invention, the sine-wave generating portion preferably
includes a first digital-to-analog converting portion operable to
convert a set of sine-wave sampling values into the first sine-wave
signal, and a second digital-to-analog converting portion operable
to convert a set of sine-wave sampling values into the second
sine-wave signal. In this case, the first and second sine-wave
signals can be generated by the first and second digital-to-analog
converters, by converting the set of sine-wave sampling values.
[0035] In the first advantageous arrangement of the sixth preferred
form of the invention, the first signal combining portion is
preferably arranged to obtain the first composite signal by
combining together the received signal received by the receiver
portion, and the first cancel signal generated by the sine-wave
synthesizing portion. In this case, the RFID-tag communication
device further comprises an analog-to-digital converting portion
operable to convert the first composite signal into an analog
signal, and the first digital-to-analog converting portion and the
analog-to-digital converting portion receive a common clock signal.
Accordingly, the first digital-to-analog converting portion
arranged to convert the set of sine-wave sampling values into the
first sine-wave signal, and the analog-to-digital converting
portion arranged to convert the first composite signal into the
analog signal can be operated in synchronization with each
other.
[0036] Preferably, the RFID-tag communication device according to
the first advantageous arrangement of the sixth preferred form of
this invention further comprises an up-converter operable to
increase a frequency of the composite sine-wave signal generated by
the sine-wave synthesizing portion, and a down-converter operable
to reduce a frequency of the first composite signal generated by
the first signal combining portion. In the presence of the
up-converter to increase the frequency of the composite sine-wave
signal, the sine-wave generating portion may be arranged to
generate the first and second sine-wave signals having relatively
low frequencies, so that these first and second sine-wave signals
having the low frequencies are combined together by the sine-wave
synthesizing portion, whereby the cost of manufacture of the
communication device can be lowered. Further, the provision of the
down-converter to reduce the frequency of the first composite
signal generated by the first signal combining portion permits the
analog-to-digital converting portion to convert the first composite
signal into the digital signal after the frequency of the first
composite signal has been reduced. In this respect, too, the cost
of manufacture of the communication device can be lowered
[0037] In the first advantageous arrangement of the sixth preferred
form of the invention, the sine-wave synthesizing portion is
preferably arranged to obtain, in addition to the first cancel
signal, a second cancel signal having an amplitude substantially
equal to an amplitude of the carrier wave, and a phase that is
reversed with respect to that of the carrier wave. In this case,
the sensitivity of reception of the reply signal can be further
improved owing to the secondary suppression of the leakage signal
according to the second cancel signal, in addition to the primary
suppression of the leakage signal according to the first cancel
signal.
[0038] In a second advantageous arrangement of the sixth preferred
form of the invention, the antenna has a plurality of antenna
elements for radio communication with the RFID tag, and the
sine-wave synthesizing portion generates the composite sine-wave
signal a phase of which has been controlled with respect to the
plurality of antenna elements, such that the generated composite
sine-wave signal is transmitted toward said RFID tag while a
directivity of the plurality of antenna elements is controlled. In
this arrangement, the composite sine-wave signal generated by the
sine-wave synthesizing portion is transmitted toward the RFID tag
while the directivity of the plurality of antenna elements is
controlled.
[0039] In the second advantageous arrangement of the sixth
preferred form of the invention, the sine-wave synthesizing portion
is preferably arranged to transmit, toward the RFID tag, the
composite sine-wave signal at least the phase of which has been
controlled, such that the direction in which the plurality of
antennas have a highest gain and which is temporarily held is
sequentially changed. In this case, a so-called "phased-array
control" of the phase of the composite sine-wave signal is effected
so as, to temporarily hold and sequentially change the direction in
which the plurality of antennas have the highest gain, so that the
reply signal transmitted from the RFID-tag can be detected by the
RFID-tag communication device, with a high degree of
sensitivity.
[0040] In the second advantageous arrangement of the sixth
preferred form of the invention, the sine-wave synthesizing portion
is preferably arranged to transmit, toward the RFID tag, the
composite sine-wave signal an amplitude and the phase of which have
been controlled, such that a sensitivity of reception by the
plurality of antennas of the reply signal transmitted from the RFID
tag is maximized. In this case, a so-called "adaptive-array
control" of the amplitude and phase of the composite sine-wave
signal is effected so as to maximize the sensitivity of reception
by the plurality of antennas of the reply signal transmitted from
the RFID tag, so that the reply signal can be detected by the
RFID-tag communication device, with a high degree of
sensitivity.
[0041] In the second advantageous arrangement of the sixth
preferred form of the invention, the sine-wave generating portion
preferably includes a first digital-analog converter operable to
convert a set of sine-wave sampling values into the first sine-wave
signal, and a second digital-analog converter operable to convert a
set of sine-wave sampling values into the second sine-wave signal.
In this case, the first and second sine-wave signals are generated
by the first and second digital-to-analog sine-wave signals, by
converting the sets of sine-wave sampling values.
[0042] In the second advantageous arrangement of the sixth
preferred form of the invention, the RFID-tag communication device
preferably further comprises an up-converter operable to increase a
frequency of the composite sine-wave signal synthesized by the
sine-wave synthesizing portion. The provision of the up-converter
to increase the frequency of the composite sine-wave signal permits
the first and second sine-wave signals to have comparatively low
frequencies, before the first and second sine-wave signals are
combined together by the sine-wave synthesizing portion to
synthesize the composite sine-wave signal, so that the cost of
manufacture of the RFID-tag communication device can be
lowered.
[0043] In a third advantageous arrangement of the sixth preferred
form of the invention, the amplitude control portion includes a
first variable attenuator or amplifier operable to control the
amplitude of the first sine-wave signal, and a second variable
attenuator or amplifier operable to control the amplitude of the
second sine-wave signal, and a polarity switching portion operable
to change polarities of the first and second sine-wave signals. In
this case, the amplitudes of the first and sine-wave signals are
controlled by the respective variable attenuators or amplifiers,
while the polarities of the first and second sine-wave signals are
controlled by the polarity switching portion. Namely, the amplitude
and polarities of the first sine-wave signal (sine signal) and the
second sine-wave signal (cosine signal) that are to be combined
together by the sine-wave synthesizing portion to synthesize the
composite sine-wave signal having the desired phase can be selected
as desired with a high degree of freedom.
[0044] In the third advantageous arrangement of the sixth preferred
form of the invention, the amplitude control portion includes a
logic circuit operable to generate a control signal in the form of
a serial signal, and a registering portion operable to convert the
serial signal into a parallel signal and generate an amplitude
control signal and a polarity control signal on the basis of the
parallel signal, the amplitude control signal and the polarity
control signal being applied to the first and second variable
attenuators or amplifiers and the polarity switching portion for
controlling the amplitudes and polarities of the first and second
sine-wave signals. The amplitude control signal generated on the
basis of the parallel signal obtained by converting the serial
signal generated by the logic circuit is applied to the variable
attenuators or amplifiers to control the amplitudes of the first
and second sine-wave signals, while the polarity control signal
similarly generated is applied to the polarity switching portion to
control the polarities of the first and second sine-wave
signals.
[0045] According to the present invention, there is also provided
an RFID-tag communication device comprising: a first-cancel-signal
output portion operable to generate a first cancel signal in the
form of a digital signal for suppressing from a received signal
received by an antenna a leakage signal that is a part of a
transmission signal which is transmitted from the antenna and which
is returned to and received by the antenna; a first-cancel-signal
D/A converting portion operable to convert the first cancel signal
generated by the first-cancel-signal output portion, into an analog
signal; a first cancel-signal control portion operable to generate
a first control signal for controlling an amplitude and/or a phase
of the first cancel signal generated by the first-cancel-signal
output portion, a first-cancel-signal attenuator operable according
to the first control signal generated by the first-cancel-signal
control portion, to attenuate the analog first cancel signal
generated by the first-cancel-signal D/A converting portion, to an
amplitude corresponding to the leakage signal; a first signal
combining portion operable to combine together the first cancel
signal which has been attenuated by the first-cancel-signal
attenuator, and the received signal, to obtain a first composite
signal. Accordingly, the present RFID-tag communication device
permits effective suppression of the leakage signal by adequately
controlling the first-cancel-signal attenuator depending upon the
level of the leakage signal, even where the leakage signal has a
comparatively low intensity. Namely, the present RFID-tag
communication device is capable of suppressing the leakage signal
with a simple arrangement, irrespective of the level or intensity
of the leakage signal.
[0046] The first-cancel-signal attenuator may be arranged to
attenuate the amplitude of the first cancel signal to a value which
is closest to but not larger than the amplitude of the leakage
signal. In this case, the amplitude of the first control signal
generated by the first-cancel-signal control portion can be made
equal to a maximum output amplitude of the first-cancel-signal D/A
converting portion, or equal to a value close to 1/2 of the maximum
output amplitude, so that the amplitude and phase of the first
cancel signal can be accurately controlled, and the leakage signal
can be effectively suppressed.
[0047] The first-cancel-signal attenuator may be arranged to
attenuate the amplitude of the first cancel signal to a value which
is larger than and closest to the amplitude of the leakage signal.
In this case, the leakage signal can be effectively suppressed.
[0048] Further, the first-cancel-signal control portion may be
arranged to generate the first control signal which causes the
amplitude of the first cancel signal generated by the
first-cancel-signal D/A converting portion, to be equal to 1/2 of
the maximum amplitude, so that the leakage signal can be
sufficiently suppressed.
[0049] The RFID-tag communication device 440 further includes: a
second-cancel-signal output portion operable to generate the second
cancel signal in the form of a digital signal for suppressing from
the received signal the leakage signal the second-cancel-signal D/A
converting portion operable to convert the second cancel signal
generated by the second-cancel-signal output portion, into an
analog signal; a second-cancel-signal output control operable to
generate a second control signal for controlling an amplitude
and/or a phase of the second cancel signal generated by the
second-cancel-signal output portion; a second-cancel-signal
attenuator operable according to the second control signal
generated by the second-cancel-signal control portion, to attenuate
the analog second cancel signal generated by the
second-cancel-signal D/A converting portion, to an amplitude
corresponding to the leakage signal; and a second signal combining
portion operable to combine together the second cancel signal which
has been attenuated by the second-cancel-signal attenuator, and the
first composite signal generated by the first signal combining
portion, to obtain a second composite signal. Accordingly, the
present RFID-tag communication device permits secondary suppression
of the leakage signal at the second signal combining portion, as
well as primary suppression of the leakage signal at the first
signal combining portion, making it possible to further improve the
signal-to-noise ratio.
[0050] The RFID-tag communication device described above may
further comprise an up-converter operable to increase a frequency
of the analog first cancel signal generated by the
first-cancel-signal D/A converting portion, and said
first-cancel-signal attenuator attenuates the first cancel signal
the frequency of which has been increased by the up-converter, by
an amount corresponding to the leakage signal. This arrangement
permits effective suppression of the leakage signal. The RFID-tag
communication device may further comprises a similar up-converter
for the second cancel signal.
[0051] The first-cancel-signal attenuator may be arranged to change
its amount of attenuation of the first cancel signal, to a selected
one of different values, so that the first-cancel-signal attenuator
can be simplified in construction, and the control to attenuate the
first cancel signal can be simplified. The second-cancel-signal
attenuator may be similarly arranged.
[0052] Further, the different values to which the amount of
attenuation of the first second cancel signal by the
first-cancel-signal attenuator is variable are multiples of 1/2, so
that the circuit arrangement of the first-cancel-signal attenuator
can be made considerably simple. Further, the control to attenuate
the first cancel signal can be simplified by a bit-shift logic
using binary digits, for example. This arrangement is applicable to
the second-cancel-signal attenuator.
[0053] The first-cancel-signal attenuator may includes a plurality
of voltage dividers in the form of a plurality of registers, and a
plurality of switches operable to selectively operate the plurality
of voltage dividers. In this case, the first-cancel-signal
attenuator is simple in construction and economical to manufacture.
The second-cancel-signal attenuator may include similar voltage
dividers and similar switches.
[0054] The first-cancel-signal attenuator may further include a
buffer amplifier functioning as a buffer device, which assures a
stable operation of the attenuator. The second-cancel-signal
attenuator may further include a similar buffer amplifier.
[0055] The first-cancel-signal D/A converting portion may use four
sampling values per one period of an output periodic function. In
this case, the noise floor can be held low, permitting a high-speed
converting operation of the first-cancel-signal D/A converting
portion, and assuring a high signal-to-noise ratio of the analog
first cancel signal which has been attenuated by the
first-cancel-signal attenuator. Similarly, the second-cancel-signal
D/A converting portion may use four sampling values per one period
of the output periodic function.
BRIEF DESCRIPTION OF THE DRAWINGS
[0056] The above and other objects, features and industrial
significance of this invention will be better understood by reading
the following detailed description of preferred embodiments of the
present invention, when considered in connection with the
accompanying drawings in which:
[0057] FIG. 1 is a view showing an arrangement of a communication
system including an RFID-tag communication device according to this
invention;
[0058] FIG. 2 is a view showing an electrical arrangement of the
RFID-tag communication device of FIG. 1 according to one embodiment
of the present invention;
[0059] FIG. 3 is a view showing a sine-wave table stored in a
function table provided in the RFID-tag communication device of
FIG. 2;
[0060] FIG. 4 is a block diagram showing an arrangement of an
RFID-tag circuit which is-provided in each RFID tag in the
communication system of FIG. 1 and which uses a sub-carrier
wave;
[0061] FIG. 5 is a block diagram showing an arrangement of an
RFID-tag circuit which is provided in each RFID tag in the
communication system of FIG. 1 and which does not use a sub-carrier
wave;
[0062] FIG. 6 is a view illustrating a waveform of a signal in the
RFID-tag communication device of FIG. 2, which signal is received
through a transmitter/receiver antenna;
[0063] FIG. 7 is a view illustrating a waveform of a signal in the
RFID-tag communication device of FIG. 2, which signal is a
down-converted signal generated from a second down-converter;
[0064] FIG. 8 is a view illustrating a waveform of a signal in the
RFID-tag communication device of FIG. 2, which signal is a first
cancel signal down-converted by a second up-converter;
[0065] FIG. 9 is a view illustrating a waveform of a signal in the
RFID-tag communication device of FIG. 2, which signal is a first
composite signal generated from a first signal combining
portion;
[0066] FIG. 10 is a view illustrating a waveform of a signal in the
RFID-tag communication device of FIG. 2, which signal is a first
composite signal down-converted by a first down-converter;
[0067] FIG. 11 is a view illustrating a waveform of a signal in the
RFID-tag communication device of FIG. 2, which signal is a second
cancel signal generated from a second cancel-signal D/A converting
portion;
[0068] FIG. 12 is a view illustrating a waveform of a signal n the
RFID-tag communication device of FIG. 2, which signal is a second
composite signal generated from a second signal combining
portion;
[0069] FIG. 13 is a view illustrating a waveform of a signal in the
RFID-tag communication device of FIG. 2, which signal is a
modulated signal generated from a modulating portion;
[0070] FIG. 14 is a part of a flow chart illustrating control
operations of a DSP of the RFID-tag communication device of FIG. 2
to suppresse a leakage signal that is a part of a transmission
signal which is generated from the communication device and
returned to and received by the communication device;
[0071] FIG. 15 is another part of the flow chart illustrating the
above-described operation of the DSP of the RFID-tag communication
device of FIG. 2;
[0072] FIG. 16 is a further part of the flow chart illustrating the
control operations of the DSP;
[0073] FIG. 17 is a still further part of the flow chart
illustrating the control operations of the DSP;
[0074] FIG. 18 is a yet further part of the flow chart illustrating
the control operations of the DSP;
[0075] FIG. 19 is another part of the flow chart illustrating the
control operations of the DSP;
[0076] FIG. 20 is a part of a flow chart alternative to the flow
chart of FIGS. 14-19;
[0077] FIG. 21 is a view for explaining frequency hopping of a
local oscillation signal in the RFID-tag communication device of
FIG. 2;
[0078] FIG. 22 is a view showing an electrical arrangement of an
RFID-tag communication device constructed according to a second
embodiment of this invention;
[0079] FIG. 23 is a view showing an electrical arrangement of an
RFID-tag communication device constructed according to a third
embodiment of this invention;
[0080] FIG. 24 is a view showing arrangements of a
transmitted-digital-signal output portion, a first-cancel-signal
output portion and a second-cancel-signal output portion of the
RFID-tag communication device of FIG. 23;
[0081] FIG. 25 is a view showing an electrical arrangement of an
RFID-tag communication device constructed according to a fourth
embodiment of this invention;
[0082] FIG. 26 is a view schematically showing an overall
arrangement of a communication system device constructed according
to a fifth embodiment of this invention;
[0083] FIG. 27 is a functional block diagram showing a functional
arrangement of an interrogator shown in FIG. 26;
[0084] FIG. 28 is a vectorial view for explaining the principle of
the present invention;
[0085] FIG. 29 is a vectorial view for explaining the principle of
the invention;
[0086] FIG. 30 is a vectorial view for explaining the principle of
the invention;
[0087] FIG. 31 is a vectorial view for explaining the principle of
the invention;
[0088] FIG. 32 is a vectorial view for explaining the principle of
the invention;
[0089] FIG. 33 is a view illustrating an example of a sine-wave
table stored in the function table;
[0090] FIG. 34 is a view illustrating another example of the
sine-wave table stored in the function table;
[0091] FIG. 35 is a view illustrating a further example of the
sine-wave table stored in the function table;
[0092] FIG. 36 is a view illustrating a still further example of
the sine-wave table stored in the function table;
[0093] FIG. 37 is a circuit diagram showing in detail a
first-composite sine-wave signal generating circuit;
[0094] FIG. 38 is a circuit diagram showing a modified form of the
composite sine-wave signal generating circuit;
[0095] FIG. 39 is a circuit diagram showing another modified form
of the composite sine-wave signal generating circuit;
[0096] FIG. 40 is a function block diagram showing a function
arrangement of an interrogator in an RFID-tag communication device
provided with an array antenna according to a sixth embodiment of
the present invention;
[0097] FIG. 41 is a circuit diagram showing in detail a
third-composite sine-wave signal generating circuit;
[0098] FIG. 42 is a circuit diagram showing in detail a modified
composite sine-wave generating circuit which is simplified in the
control signal received by the circuit;
[0099] FIG. 43 is a circuit diagram showing a composite sine-wave
signal generating circuit which is obtained by applying the
composite sine-wave signal generating circuit of FIG. 42 to the
circuit of FIG. 40;
[0100] FIG. 44 is a view showing an arrangement of an RFID-tag
communication device constructed according to a seventh embodiment
of this invention;
[0101] FIG. 45 is a view showing an arrangement of an attenuator
provided in the RFID-tag communication device of FIG. 44;
[0102] FIG. 46 is a view indicating a relationship between a
leakage carrier wave level and an amount of suppression;
[0103] FIG. 47 is a part of a flow chart illustrating control
operations of a DSP of the RFID-tag communication device of FIG. 44
to suppress a leakage signal that is a part of the transmission
signal transmitted from the RFID-tag communication device and
returned to the communication device, this part corresponding to
that of FIG. 14;
[0104] FIG. 48 is another part of the flow chart illustrating the
above-described control operations of the DSP of the RFID-tag
communication device of FIG. 47, this part corresponding to that of
FIG. 15;
[0105] FIG. 49 is a further part of the flow chart illustrating the
above-described control operations of the DSP, this part
corresponding to that of FIG. 17;
[0106] FIG. 50 is a still further part of the flow chart
illustrating the control operations of the DSP, this part
corresponding to that of FIG. 18;
[0107] FIG. 51 is a view showing an arrangement of an RFID-tag
communication device constructed according to an eighth embodiment
of this invention; and
[0108] FIG. 52 is a view showing an arrangement of an RFID-tag
communication device constructed according to a ninth embodiment of
this invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0109] The preferred embodiments of the present invention will be
described by reference to the accompanying drawings.
Embodiment 1
[0110] Reference is first made to FIG. 1 showing an RFID-tag
communication system 10 to which the present invention is
applicable and which includes an RFID-tag communication device 12
constructed according to a first embodiment of this invention, and
a plurality of radio-frequency identification tags (hereinafter
abbreviated as "RFID tags") 14, more specifically, four RFID tags
14a, 14b, 14c and 14d. In the present RFID-tag communication system
10, the RFID-tag communication device 12 serves as an interrogator,
while the RFID tags 14 serve as transponders. Namely, the RFID-tag
communication device 12 is arranged to transmit a transmission
signal in the form of a carrier wave F.sub.c1, while the RFID tags
14a, 14b, 14c, 14d are arranged to receive the carrier wave
F.sub.c1, modulate the received carrier wave F.sub.c1 into reply
signals in the form of reflected waves F.sub.r1, F.sub.r2, F.sub.r3
and F.sub.r4 (hereinafter collectively referred to as "reflected
waves F.sub.rf) on the basis of suitable information, and transmit
the reflected waves F.sub.rf in reply to the carrier wave F.sub.c1.
The RFID-tag communication device 12 is arranged to demodulate the
received reflected waves F.sub.rf. Thus, radio communication is
effected between the RFID-tag communication device 12 and the RFID
tags.
[0111] Referring next to FIG. 2 showing an electrical arrangement
of the RFID-tag communication device 12, this device 12 is operable
to effect radio communication with the RFID tags 14, for
implementing at least one of information writing and reading on and
from the RFID tags 14. To this end, the RFID-tag communication
device 12 includes a DSP (digital signal processor) 16, and a
transmitter/receiver circuit 18. The DSP 16 is operable to output a
digital signal as the transmission signal, and perform digital
signal processing operations such as an operation to demodulate the
reply signals received from the RFID tags 14. The
transmitter/receiver circuit 18 is operable to convert the digital
transmission signal received from the DSP 16, into an analog signal
serving as the carrier wave F.sub.c1, and to convert the reflected
signals F.sub.rf received from the RFID tags 14, into a digital
signal, which is applied to the DSP 16.
[0112] The DSP 16 is a so-called microcomputer system which
incorporates a CPU, a ROM and a RAM and which operates to perform
signal processing operations according to control programs stored
in the ROM, while utilizing a temporary data storage function of
the RAM. The DSP 16 includes, as functional elements, a
digital-transmission-signal output portion 20, a modulating portion
22, a first-cancel-signal output portion 24, a first-cancel-signal
control portion 26, a second-cancel-signal output portion 28, a
second-cancel-signal control portion 30, a demodulating portion 32,
a direct-current-component detecting portion 34, a
received-signal-amplitude detecting portion 36, a
first-composite-signal amplitude detecting portion 38, and a
function table 40. The digital-transmission-signal output portion
20 is arranged to generate the digital transmission signal to be
transmitted to the RFID tags 14. The modulating portion 22 is
arranged to modulate the digital transmission signal generated by
the digital-transmission-signal output portion 20, on the basis of
a suitable command signal. The first-cancel-signal output portion
24 is arranged to generate a first cancel signal in the form of a
digital signal, on the basis of the digital transmission signal.
The first-cancel-signal control portion 26 is arranged to control
at least one of an amplitude A1 and a phase .phi.C1 of the first
cancel signal generated by the first-cancel-signal output portion
24. The second-cancel-signal output portion 28 is arranged to
generate a second cancel signal in the form of a digital signal, on
the basis of the digital transmission signal. The
second-cancel-signal control portion 30 is arranged to control at
least one of an amplitude A2 and a phase .phi.C2 of the second
cancel signal generated by the second-cancel-signal output portion
28. The demodulating portion 32 is arranged to demodulate a
received signal received through a transmitter/receiver antenna 52
(which will be described). The direct-current-component detecting
portion 34 is arranged to detect a direct-current component (DC
component) of the demodulated signal generated by the demodulating
portion 32. The received-signal-amplitude detecting portion 36 is
arranged to detect an amplitude AR of the above-indicated received
signal. The first-composite-signal-amplitude detecting portion 38
is arranged to detect an amplitude AM1 of a first composite signal
generated by a first signal combining portion 58 (which will be
described). The function table 40 is provided to store sampling
values corresponding to respective different phases at
predetermined sampling points.
[0113] The function table 40 preferably stores a sine-wave or
cosine-wave table of the sampling values corresponding to the
different phases at the predetermined sampling points. FIG. 3 shows
an example of the sine-wave table, which indicates two
predetermined values sin .phi. and sin .phi.(.pi.+0.5)
corresponding to an initial phase .phi.. For example, the sine-wave
table indicates sin .phi.=0, and sin (.phi.+0.5.pi.)=1 which
correspond to .phi.=0, and indicates sin .phi.=0.58779, and sin
(.phi.+0.5.pi.)=0.80902 which correspond to .phi.=0.2#. Since sin
(.phi.+.pi.)=sin .phi., it is possible to read out, from the
sine-wave table, sin (.phi.+.pi.) and sin (.phi.+1.5.pi.) on the
basis of the above-indicated two values. For instance, successive
values "0,1,0,-1, 0,1,0,-1,0,1,0,-1 . . . " corresponding to
.phi.=0 are read out from the sine-wave table, and successive
values "0.58779, 0.80902, -0.58779, -0.80902, 0.58779, 0.80902,
-0.58779, -0.80902, 0.58779, 0.80902, -0.58779, -0.80902 . . . "
corresponding to .phi.=0.2.pi. are read out from the sine-wave
table. On the basis of these successive values, a suitable
sine-wave signal is generated. The phase of the sine-wave signal
can be changed by changing the positions of the sine-wave table in
the function table 40 from which the successive values are read
out, and the amplitude of the sine-wave signal can be changed by
multiplying the read-out sine-wave signal by a desired control
value. Therefore, the above-described digital-transmission-signal
output portion 20, first-cancel-signal output portion 24 and
second-cancel-signal output portion 28 can generate the desired
sine-wave signals.
[0114] The transmitter/receiver circuit 18 includes a
transmission-signal D/A converting portion 42, a
local-analog-oscillation-signal output portion 44, a first
up-converter 46, a first amplifying portion 48, a
transmission/reception separator 50, a transmitter/receiver antenna
52, a first-cancel-signal D/A converting portion 54, a second
up-converter 56, a first signal combining portion 58, a second
amplifying portion 60, a first down-converter 62, a
second-cancel-signal D/A converting portion 64, a second signal
combining portion 66, a third amplifying portion 68, a
first-composite-signal A/D converting portion 70, a
second-composite-signal A/D converting portion 72, a second
down-converter 74, a received-signal A/D converting portion 76, and
a clock-signal output portion 78. The transmission-signal D/A
converting portion 42 is arranged to convert the digital
transmission signal generated by the modulated portion 22, into an
analog signal. The local-analog-oscillation-signal output portion
44 is arranged to generate a suitable local analog oscillation
signal. The first up-converter 46 is arranged to increase the
frequency of the analog transmission signal received from the
above-described transmission-signal D/A converting portion 42, by
an amount corresponding to the frequency of the local analog
oscillation signal generated by the local-analog-oscillation-signal
output portion 44. The first amplifying portion 48 is arranged to
amplify the transmission signal generated by the first up-converter
46. The transmission/reception separator 50 is arranged to apply
the transmission signal generated by the first amplifying portion
48, to a transmitter/receiver antenna 52, and to apply the reply
signals (the received signals containing a leakage signal) received
from the above-described RFID tags 14 through transmitter/receiver
antenna 52, to the first-signal combining portion 58 and the second
down-converter 74. The leakage signal is a part of the transmission
signal which is transmitted from the antenna 52 of the RFID-tag
communication device 12 and which is returned to and received by
the antenna 52. The transmitter/receiver antenna 52 is arranged to
transmit, as the carrier wave F.sub.c1, the transmission signal
received from the transmission/reception separator 50, and to
receive the reflected waves F.sub.rf from the RFID tags 14 and
apply the received reflected waves F.sub.rf to the
transmission/reception separator 50. The first-cancel-signal D/A
converting portion 54 is arranged to convert the first cancel
signal generated by the above-described first-cancel-signal output
portion 24, into an analog signal. The second up-converter 56 is
arranged to increase the frequency of the first cancel signal
converted into the analog signal by the first-cancel-signal D/A
converting portion 54, by an amount corresponding to the frequency
of the local analog oscillation signal generated by the
above-described local-analog-oscillation-signal output portion 44.
The first signal combining portion 58 is arranged to combine
together the first cancel signal generated by the second up
converter 56, and the above-described received signals received
from the transmitter/receiver antenna 52 through the
transmission/reception separator 50, obtain a first composite
signal. The second amplifying portion 60 is arranged to amplify the
first composite signal generated by the first signal combining
portion 58, for increasing its amplitude AM1. The first
down-converter 62 is arranged to reduce the frequency of the first
composite signal generated by the second amplifying portion 60, by
an amount corresponding to the frequency of the local analog
oscillation signal generated by the above-described
local-analog-oscillation-signal output portion 44. The
second-cancel-signal D/A converting portion 64 is arranged to
convert the second cancel signal generated by the above-described
second-cancel-signal output portion 28, into an analog signal. The
second signal combining portion 66 is arranged to combine together
(and amplify, when needed) the second cancel signal converted by
the second-cancel-signal D/A converting portion 64 into the analog
signal, and the first composite signal generated by the first
down-converter 62, to obtain a second composite signal. The third
amplifying portion 68 is arranged to amplify the first composite
signal generated by the first down-converter 62, for increasing its
amplitude AM1. The first-composite-signal A/D converting portion 70
is arranged to convert the first composite signal received from the
third amplifying portion 68, into a digital signal, and to apply
the digital first composite signal to the above-described
first-composite-signal-amplitude detecting portion 38. The
second-composite-signal A/D converting portion 72 is arranged to
convert the second composite signal received from the third
amplifying portion 66, into a digital signal, and to apply the
digital second composite signal to the above-described demodulating
portion 32. The second down-converter 74 is arranged to reduce the
frequency of the above-described received signals received through
the transmission/reception separator 50, by an amount corresponding
to the frequency of the local analog oscillation signal generated
by the local-analog-oscillation-signal output portion 44. The
received-signal A/D converting portion 76 is arranged to convert
the received signals received from the second down-converter 74,
into digital signals, and to apply the digital received signals to
the above-described received-signal-amplitude detecting portion 36.
The clock-signal output portion 78 is arranged to generate a
suitable clock signal. Preferably, the clock-signal output portion
78 applies the clock signal to the above-described received-signal
D/A converting portion 42, and applies the same clock signal to at
least one of, and desirably all of the above-described
first-cancel-signal D/A converting portion 54, second-cancel-signal
D/A converting portion 64, first-composite-signal A/D converting
portion 70, second-composite-signal A/D converting portion 72 and
received-signal A/D converting portion 76. Preferably, the
above-described received-signal A/D converting portion 76 uses a
converter having a smaller number of bits, than a converter used by
the first-composite-signal A/D converting portion 70, etc. This
converter used by the received-signal A/D converting portion 76 has
an advantage that a component of the received signals which relates
to the modulation by the RFID tags 14 can be ignored. It is noted
that the local-analog-oscillation-signal output portion 44 uses an
oscillator operable to generate a frequency in the neighborhood of
900 MHz or 2.4 GHz. The transmission/reception separator 50
generally uses a circulator or a directional coupler.
[0115] Referring next to the block diagram of FIG. 4, there is
shown an arrangement of an RFID-tag circuit 15A of each of the RFID
tags 14. This RFID-tag circuit 15A includes an antenna portion 80
for receiving the transmission signal in the form of the carrier
wave Fi from the RFID-tag communication device 12 and transmitting
the reply signal in the form of the reflected wave F.sub.rf in
reply to the received transmission signal, a
modulating/demodulating portion 82 connected to the antenna portion
80 and operable to modulate and demodulate the received signals,
and a digital circuit portion (ID-circuit portion) 84 operable to
perform digital signal processing operations. The digital circuit
portion 84 includes a control portion 86 operable to control the
operation of the RFID-tag circuit 15A, by using the carrier wave
Fc1 received from the antenna portion 80, as an energy source, a
sub-carrier wave oscillating portion 88 operable to generate a
sub-carrier wave, and a sub-carrier wave modulating portion 90
operable to modulate the sub-carrier wave generated by the
sub-carrier wave oscillating portion 86, on the basis of a suitable
information signal received through the control portion 86.
[0116] There will be described an operation of the communication
system constructed as described above. Initially, the
digital-transmission-signal output portion 20 of the RFID-tag
communication device 12 generates the digital transmission signal
on the basis of the function table stored in the function table 40.
For instance, this transmission signal is a signal generated by
sampling a sine wave. The digital transmission signal generated by
the digital-transmission-signal output portion 20 is modulated by
the modulating portion 22. Then, the modulated digital transmission
signal generated by the modulating portion 22 is converted by the
transmission-signal D/A converting portion 42 into an analog
signal. The frequency of the analog transmission signal generated
by the transmission-signal D/A converting portion 42 is then
increased by the first up-converter 46, by an amount corresponding
to the frequency of the local analog oscillation signal generated
by the local-analog-oscillation-signal output portion 44. Then, the
amplitude of the transmission signal generated by the first
up-converter 46 is increased by the first amplifying portion 48.
The transmission signal which has been amplified by the first
amplifying portion 48 is applied to the transmitter/receiver
antenna 53 through the transmission/reception separator 50, and is
transmitted as the carrier wave F.sub.c1 from the
transmitter/receiver antenna 82 toward the RFID tags 14.
[0117] The carrier wave Fc1 transmitted from the antenna 52 is
received by the antenna portion 80 of the RFID tags 14, and
demodulated by the modulating portion 82. A portion of the received
carrier wave F.sub.c1 is rectified by a rectifying portion (not
shown), and a sub-carrier wave is generated by the sub-carrier
oscillating portion 88, with the rectified carrier wave F.sub.c1
used as an energy source. The sub-carrier wave generated by the
sub-carrier oscillating portion 88 is subject to primary modulation
by the sub-carrier modulating portion 90 on the basis of suitable
information signal received through the control portion 86. The
carrier wave F.sub.c1 is subjected to secondary modulation by the
modulating/demodulating portion 82, according to the sub-carrier
wave subjected to the primary modulation by the sub-carrier
modulating portion 90. The carrier wave F.sub.c1 subjected to the
secondary modulation is transmitted as the reflected wave F.sub.rf
from the antenna portion 80, toward the RFID-tag communication
device 12. The RFID tags 14 may have a modified RFID-tag circuit
15B as shown in FIG. 5, which does not use a sub-carrier wave. In
this case, the signal supplied from the control portion 86B to the
modulating/demodulating portion 82B and transmitted from the RFID
tag 14 as the reply signal must be modulated by ASK (which may use
an FSK signal) or PSK.
[0118] The reflected wave F.sub.rf transmitted from each RFID tag
14 is received by the antenna 52 of the RFID-tag communication
device 12, and applied as the received signal to the first signal
combining portion 58 and the second down-converter 74 through the
transmission/reception separator 50. At this time, a leakage signal
that is a part of the transmission signal transmitted from the
RFID-tag communication device 12 and returned to the communication
device 12 through the transmission/reception separator 50 may be
applied to the first signal combining portion 58 and second
down-converter 74, together with the received signal. FIG. 6
illustrates a waveform of the received signal received by the first
signal combining portion 58. It will be understood from FIG. 6 that
the amplitude-modulated component is very small since the amplitude
of the leakage signal is considerably larger than that of the
reflected wave (reply signal). The frequency of the received signal
received by the second down-converter 74 is reduced by the amount
corresponding to the frequency of the local analog oscillation
signal generated by the local-analog-oscillation-signal output
portion 44. FIG. 7 illustrates a waveform of the received signal
the frequency of which has been reduced by the second
down-converter 74. The received signal the frequency of which has
been reduced by the second down converter 74 is converted by the
received-signal A/D converting portion 76, into a digital signal
which is applied to the received-signal-amplitude detecting portion
36, so that the amplitude AR of the received signal is detected by
the received-signal-amplitude detecting portion 36. An output of
the received-signal-amplitude detecting portion 36 is applied to
the first-cancel-signal control portion 26.
[0119] The first-cancel-signal control portion 26 determines the
phase .phi.C1 and the amplitude A1 of the first cancel signal, and
the first-cancel-signal output portion 24 generates the first
cancel signal in the form of a digital signal on the basis of the
function table stored in the function table 40. Preferably, the
amplitude A1 of the first cancel signal is determined by the
amplitude AR of the received signal received from the
received-signal-amplitude detecting portion 36. For example, the
amplitude A1 of the first cancel signal which has been converted
into the analog signal by the D/A converting portion 54 and the
frequency of which has been increased by the second up-converter 56
is determined to be equal to the amplitude of the received signal
received by the first signal combining portion 58. The first cancel
signal generated by the first-cancel-signal output portion 24 is
converted by the first-cancel-signal D/A converting portion 54 into
the analog signal, and the frequency of the analog first cancel
signal generated by the D/A converting portion 54 is increased by
the second up-converter 56 by the amount corresponding to the
frequency of the local analog oscillation signal generated by the
local-analog-oscillation-signal output portion 44. FIG. 8
illustrates a waveform of the first cancel signal the frequency of
which has been increased by the second up-converter 56. The first
cancel signal the frequency of which has been increased by the
second up-converter 56 and the received signal received by the
first signal combining portion 58 from the transmission/reception
separator 50 are combined together by the first signal combining
portion 58 to obtain the first composite signal which does not
include the leakage signal that is a part of the transmission
signal which is transmitted from the RFID-tag communication device
12 and which is returned to and received by the communication
device 12. Namely, the leakage signal is partially suppressed from
the received signal received by the first signal combining portion
58. FIG. 9 illustrates a waveform of the first composite signal
generated by the first signal combining portion 58. In FIG. 9, the
amplitude of the first composite signal is exaggerated. While the
amplitude of the first composite signal is smaller than that shown
in FIG. 6 by an amount corresponding to the amplitude of the
leakage signal, the first composite signal has the waveform as
shown in FIG. 9 after this first composite signal is amplified by
the second amplifying portion 60, because the amplitude-modulated
component remains unchanged. The amplitude-modulated component of
the first composite signal of FIG. 9 is larger than the other
component, since a most of the leakage signal has been suppressed
from the received signal of FIG. 6. Since the intensity or strength
of the reply signal transmitted from the RFID tags 14 is higher
than that of the leakage signal, a non-negligible portion of the
leakage signal remains in the first composite signal. The amplitude
of the first composite signal generated by the first signal
combining portion 58 is amplified by the second amplifying portion
60, by a gain G1. Where the amplitude A1 and the phase .phi.C1 of
the first cancel signal are suitably determined, the intensity of
the first cancel signal is relatively low, and the intensity of the
input to the first down-converter 62 is accordingly low, so that
the gain G1 of the second amplifying portion 60 is accordingly
increased. A gain G2 of the above-described third amplifying
portion 68 has a predetermined initial value. Then, the frequency
of the first composite signal generated by the second amplifying
portion 60 is reduced by the first down-converter 62 by the amount
corresponding to the frequency of the local analog oscillation
signal generated by the local-analog-signal output portion 44, and
the first composite signal the frequency of which has been reduced
is applied to the second signal combining portion 66 and the third
amplifying portion 68.
[0120] FIG. 10 illustrates a waveform of the first composite signal
the frequency of which has been reduced by the first down-converter
62. The frequency of the clock signal generated by the clock-signal
output portion 78 is preferably four times the frequency of the
first composite signal (intermediate frequency signal) the
frequency of which has been reduced, or a multiple of the frequency
four times that of this intermediate frequency signal. The
amplitude of the first composite signal applied to the third
amplifying portion 68 is amplified by the gain G2. The amplitude of
the first composite signal generated by the first down-converter 62
decreases as the suppression of the leakage signal progresses.
Accordingly, the gain G2 of the third amplifying portion 68 is
preferably increased as the suppression of the leakage signal
progresses. The first composite signal generated by the third
amplifying portion 68 is converted by the first-composite-signal
A/D converting portion 70, into the digital signal, and the digital
first composite signal is applied to the
first-composite-signal-amplitude detecting portion 38, so that the
amplitude AM1 of the first composite signal is detected by the
first-composite-signal-amplitude detecting portion 38. The output
of the first-composite-signal-amplitude detecting portion 38 is
applied to the first-cancel-signal control portion 26 and the
second-cancel-signal control portion 30. Preferably, the phase
.phi.C1 of the first cancel signal is controlled by the
first-cancel-signal control portion 26, on the basis of the
amplitude AM1 of the first composite signal, such that the
amplitude AM1 is minimized, and preferably such that the mean value
of the amplitude AM1 of the first composite signal is
minimized.
[0121] In the meantime, the phase .phi.C2 and the amplitude AM2 of
the second cancel signal described above are determined by the
second-cancel-signal control portion 30. The second cancel signal
in the form of a digital signal is generated by the
second-cancel-signal output portion 28, on the basis of the
function table stored in the function table 40. Preferably, the
amplitude AM2 of the second cancel signal is determined on the
basis of the amplitude AM1 of the first composite signal received
from the first-composite-signal-amplitude detecting portion 38.
Preferably, the amplitude A2 of the second cancel signal which has
been converted into the analog signal by the D/A converting portion
64 is determined to be equal to the amplitude of the first
composite signal the frequency of which has been reduced by the
first down-converter 62 and which has been received the second
signal combining portion 66. The second cancel signal generated by
the second-cancel-signal output portion 28 is converted by the
second-cancel-signal D/A converting portion 64 into the analog
signal. FIG. 11 illustrates a waveform of the second cancel signal
generated by the second-cancel-signal output portion 64. The second
cancel signal generated by the second-cancel-signal D/A converting
portion 64 and the first composite signal the frequency of which
has been reduced by the first down-converter 62 are combined
together by the second signal combining portion 66, to obtain the
second composite signal which does not include the leakage signal.
Namely, the leakage signal has been partially suppressed from the
first composite signal received by the second signal combining
portion 66. FIG. 12 illustrates a waveform of the second composite
signal generated by the second signal combining portion 66. The
amplitude-modulated component of the second composite signal shown
in FIG. 12 is larger than the other component, to a larger extent
than in the received signal of FIG. 6 and the first composite
signal of FIG. 9. It will thus be understood that the leakage
signal is very small in the second composite signal. The second
composite signal generated by the second signal combining portion
66 is converted by the second-composite-signal A/D converting
portion 72 into a digital signal, and the digital second composite
signal is applied to the first-cancel-signal control portion 26 and
the demodulating portion 32. The second composite signal is
demodulated by the demodulating portion 32, whereby the information
signal received from the RFID tag 14 is read. Preferably, the phase
.phi.C1 of the first cancel signal is controlled by the
first-cancel-signal control portion 26, on the basis of the second
composite signal. Preferably, the second signal combining portion
66 functions as a differential amplifier, and a gain G3 of the
second signal combining portion 66 is controlled such that an input
voltage of the second-composite-signal A/D converting portion 72
coincides with a desired value.
[0122] FIG. 13 illustrates a waveform of the demodulated signal
generated by the demodulating portion 32. The
direct-current-component detecting portion 34 is arranged to detect
a direct current component of the demodulated signal of FIG. 13,
and an output of the direct-current-component detecting portion 34
is applied to the second-cancel-signal control-portion 30. This
direct current component corresponds to the leakage signal received
by the RFID-tag communication device 12. Preferably, the phase
.phi.C2 of the second cancel signal is controlled by the
second-cancel-signal control portion 30, so as to minimize an
amplitude D2 of the direct current component. The arrangement of
the RFID-tag communication device 12 described above permits
effective suppression of the leakage signal received by the
communication device 12, and assures highly sensitive communication
with the RFID tags 14.
[0123] Referring to the flow charts of FIGS. 14-19, there will be
described an operation of the DSP 16 of the RFID-tag communication
device 12 to suppresse the leakage signal by the communication
device 12. A control routine shown in the flow charts is executed
with an extremely short cycle time of about several milliseconds to
about several tens of milliseconds.
[0124] The control routine is initiated with step S1 of FIG. 14 to
reset the phase .phi.C1 of the first cancel signal and .phi.C2 of
the second cancel signal to "0", and set the gain G1 of the second
amplifying portion 60 to "1". Then, the control flow goes to step
S2 to determine whether a command signal should be transmitted
toward the RFID tags 14. The transmission of the command signal is
requested in an upper-order control routine (not shown). If an
affirmative decision is obtained in step S2, the control flow goes
to step S3 corresponding to the modulating portion 22 to modulate
the above-described transmission signal according to the command
signal. Then, step S5 and the following steps are implemented. If a
negative decision is obtained in step S2, the control flow goes to
step S4 to inhibit the modulation of the transmission signal, and
then goes to the step S5 and the flowing steps. The step S5 is
provided to read the received signal which has been converted into
the digital signal by the received-signal A/D converting portion
76. In the next step S6 corresponding to the
received-signal-amplitude detecting portion 36, the amplitude AR of
the received signal read in the step S5 is detected. In the next
step S7, the amplitude A1 of the first cancel signal is determined.
Then, the control flow goes to step S8 of FIG. 15 and the following
steps.
[0125] In the step S8 of FIG. 15, variables "i", "k" and "X" are
reset to "0". The control flow then goes to step S9 to read out
function values from the function table 40. In the next step S10,
the function values read out in the step S9 are multiplied by the
amplitude A1 determined in the step S7. The control flow then goes
to step S11 to read the first composite signal which has been
converted into the digital signal by the first-composite-signal A/D
converting portion 70. In the next step S12, the amplitude AM1 of
the first composite signal detected in the step S11 is detected.
Then, the control flow goes to step S13 to determine whether the
amplitude AM1 of the first composite signal detected in the step
S12 is equal to or lower than a first threshold value. That is, an
optimum range of the amplitude AM1 in which the input voltage of
the first-composite-signal A/D converting portion 70 is adequate is
defined by the first threshold value that is a lower limit, and a
second threshold value that is an upper limit. If the input voltage
is initially lower than the first threshold value or lower limit,
the gain G2 of the third amplifying portion 68 should be increased.
If an affirmative decision is obtained in the step S13, therefore,
the control flow goes to step S14 to determine whether the gain G2
of the third amplifying portion 68 is set at the maximum value. If
a negative decision is obtained in the step S13, the control flow
goes to step S13' to determine whether the amplitude AM1 of the
first composite signal detected in the step S12 is equal to or
higher than the second threshold value. If a negative decision is
obtained in the step S13', this means that the input voltage of the
first-composite-signal A/D converting portion 70 is adequate. In
this case, the control flow goes to step S21 of FIG. 16 and the
following steps. If an affirmative decision is obtained in the step
S13', that is, if the input voltage of the first-composite-signal
A/D converting portion 70 is higher than the second threshold value
or upper limit, the control flow goes to step S18 to determine
whether the gain G2 is set at the minimum value. If an affirmative
decision is obtained in the step S14, that is, the gain G2 of the
third amplifying portion 68 is set at the maximum value, the
control flow goes to the step S21 of FIG. 16 and the following
steps. If a negative decision is obtained in the step S14, that is,
if the gain G2 of the third amplifying portion 68 is not set at the
maximum value, the control flow goes to step S15 to divide the
variable "X" by the gain G2. The step S15 is followed by step S16
to add a predetermined value dG to the gain G2, and step S17 to
multiply the variable "X" by the gain G2. Then, the control flow
goes back to the step S11. If an affirmative decision is obtained
in the step S18, that is, if the gain G2 of the third amplifying
portion 68 is set at the minimum value, the control flow goes to
the step S21 of FIG. 16 and the following steps. If a negative
decision is obtained in the step S18, the control flow goes to step
S19 to divide the variable "X" by the gain G2, and to step S20 to
subtract the predetermined value dG from the gain G2. Then, the
control flow goes to the step S17 and the following steps. Thus,
the gain G2 of the third amplifying portion 68 is controlled such
that the amplitude AM1 of the first composite signal (the input
voltage of the first-composite-signal A/D converting portion 70) is
held within the optimum range, so that the communication of the
RFID-tag communication device 12 with the RFID tags 14 can be
effected with high sensitivity, even when the amplitude of the
first composite signal generated by the first signal combining
portion 58 is reduced by the suppression of the leakage signal.
[0126] The step S21 of FIG. 16 is provided to determine whether the
variable "i" is set at "0". If an affirmative decision is obtained
in the step S21, the control flow goes to step S22 to set the
variable "X" to AM1, and set the variable "i" to "1". Then, the
control flow goes to step S23 to add a predetermined value d.phi.
to the phase .phi.C1 of the first cancel signal, and to the step S9
of FIG. 15 and the following steps. If a negative decision is
obtained in the step S21, the control flow goes to step S24 to
determine whether the variable "X" is larger than the amplitude
AM1. If an affirmative decision is obtained in the step S24, the
control flow goes to step S25 to set the variable "X" to AM1, and
to the step S23. If a negative decision is obtained in the step
S24, the control flow goes to step S26 to determine whether the
variable "k" is set at "0". If an affirmative decision is obtained
in the step S26, the control flow goes to step S27 to set the
variable "k" to "1", and set the predetermined value d.phi. to
-d.phi., and then goes to the step S23. If a negative decision is
obtained in the step S26, the control flow goes to step S28 to set
the gain G2 to "1", and to step S29 to determine the phase .phi.C1
of the first cancel signal. The step S29 is followed by step S30 of
FIG. 17 and the following steps. Thus, the phase .phi.C1 of the
first cancel signal is controlled so as to minimize the amplitude
AM1 of the first composite signal.
[0127] In the step S30 of FIG. 17, the predetermined value dG is
added to the gain G1 of the second amplifying portion 60. Then,
step S31 is implemented to read the first composite signal which
has been converted into the digital signal by the
first-composite-signal A/D converting portion 70. Step S32 is then
implemented to detect the amplitude AM1 of the first composite
signal read in the step S31. The control flow then goes to step S33
to determine whether the amplitude AM1 of the first composite
signal detected in the step S32 is set equal to a predetermined
value. If an affirmative decision is obtained in the step S33, the
control flow goes to step S35 to determine the amplitude AM2 of the
second cancel signal, and to step S36 of FIG. 18 and the following
steps. If a negative decision is obtained in the step S33, the
control flow goes to step S34 to determine whether the gain G1 of
the second amplifying portion 60 is set at the maximum value. If an
affirmative decision is obtained in the step S34, the control flow
goes to the step S35 and the following steps. The operation of the
DSP 16 described above by reference to the flow charts of FIGS.
14-17 permits high-frequency amplification by the second amplifying
portion 60 of the received signal the amplitude-modulated component
of which is increased as a result of suppression of the leakage
signal. Accordingly, the present RFID-tag communication device 12
permits detection of the reply signals with high sensitivity, with
a reduced influence of the noise.
[0128] In the step S36 of FIG. 18, the variables "i", "k" and "Y"
are reset to "0". Then, step S37 is implemented to read out the
function values from the function table 40. The control flow then
goes to step S38 to multiply the function values read out in the
step S37, by the amplitude A2 determined in the step S35. Step S39
is then implemented to determine whether the demodulated signal
output of the demodulating portion 32 is present. If a negative
decision is obtained in the step S39, the control flow goes to step
S40 determine whether a predetermined time has elapsed. If an
affirmative decision is obtained in the step S40, this means that
the RFID tags 14 have not transmitted the reply signals. In this
case, the control flow goes back to the step S2 of FIG. 14 and the
following steps. If a negative decision is obtained in the step
S40, the control flow goes back to the step S39. If an affirmative
decision is obtained in the step S39, that is, if the demodulated
signal output of the demodulating portion 32 is present, the
control flow goes to step S41 corresponding to the demodulating
portion 32, to read the demodulated second composite signal
generated by the demodulating portion 32. Then, step S42
corresponding to the direct-current-component detecting portion 34
is implemented to detect the amplitude D2 of the direct current
component of the demodulated second composite signal. The control
flow then goes to step S43 to detect a maximum amplitude AB.sub.max
of the modulated second composite signal, and to step S44 to
determine whether the maximum amplitude AB.sub.max of the
demodulated signal detected in the step S43 is equal to or lower
than a first threshold value (which is different from the first
threshold value used in the step S13 of FIG. 15). That is, an
optimum range of the amplitude AB.sub.max in which the input
voltage of the second-composite-signal A/D converting portion 72 is
adequate is defined by the first threshold value that is a lower
limit, and a second threshold value that is an upper limit. If the
input voltage is initially lower than the first threshold value or
lower limit, the gain G3 of the second signal combining portion 66
should be increased. If an affirmative decision is obtained in the
step S44, therefore, the control flow goes to step S45 to determine
whether the gain G3 of the second signal combining portion 66 is
set at the maximum value. If a negative decision is obtained in the
step S45, the control flow goes to step S44' to determine whether
the amplitude AB.sub.max of the modulated second composite signal
detected in the step S43 is equal to or higher than the second
threshold value (which is different from the second threshold value
used in the step S13'of FIG. 15). If a negative decision is
obtained in the step S44', this means that the input voltage of the
second-composite-signal A/D converting portion 72 is adequate. In
this case, the control flow goes to step S52 of FIG. 19 and the
following steps. If an affirmative decision is obtained in the step
S44', that is, if the input voltage of the second-composite-signal
A/D converting portion 72 is higher than the second threshold value
or upper limit, the control flow goes to step S49 to determine
whether the gain G3 is set at the minimum value. If an affirmative
decision is obtained in the step S45, that is, the gain G3 of the
second signal combining portion 66 is set at the maximum value, the
control flow goes to the step S52 of FIG. 19 and the following
steps. If a negative decision is obtained in the step S45, that is,
if the gain G3 of the second signal combining portion 66 is not set
at the maximum value, the control flow goes to step S46 to divide
the variable "Y" by the gain G3. The step S46 is followed by step
S47 to add a predetermined value dG to the gain G3, to increase the
amplifying factor of the second signal combining portion 66, and
step S48 to multiply the variable "Y" by the gain G3. Then, the
control flow goes back to the step S39. If an affirmative decision
is obtained in the step S49, that is, if the gain G3 of the second
signal combining portion 66 is set at the minimum value, even where
the input voltage of the second-composite-signal A/D converting
portion 72 is higher than the upper limit, the control flow goes to
the step S52 of FIG. 16 and the following steps. If a negative
decision is obtained in the step S49, that is, if the gain G3 of
the second signal combining portion 66 is not set at the minimum
value, the control flow goes to step S50 to divide the variable "Y"
by the gain G3, and to step S51 to subtract the predetermined value
dG from the gain G3, to reduce the amplifying factor of the second
signal combining portion 66. Then, the control flow goes to the
step S48 and the following steps.
[0129] The step S52 of FIG. 19 is provided to determine whether the
variable "i" is set at "0". If an affirmative decision is obtained
in the step S52, the control flow goes to step S53 to set the
variable "Y" to D2, and set the variable "i" to "1". Then, the
control flow goes to step S54 to add a predetermined value d.phi.
to the phase .phi.C2 of the second cancel signal, and to the step
S37 of FIG. 18 and the following steps. If a negative decision is
obtained in the step S52, the control flow goes to step S55 to
determine whether the variable "Y" is larger than the amplitude D2
of the direct current component of the demodulated second composite
signal. If an affirmative decision is obtained in the step S55, the
control flow goes to step S56 to set the variable "Y" to D2, and to
the step S54. If a negative decision is obtained in the step S55,
the control flow goes to step S57 to determine whether the variable
"k" is set at "0". If an affirmative decision is obtained in the
step S57, the control flow goes to step S58 to set the variable "k"
to "1", and set the predetermined value d.phi. to -d.phi., and then
goes to the step 54. If a negative decision is obtained in the step
S57, the control flow goes to step S59 to determine the phase
.phi.C2 of the second cancel signal. The step S59 is followed by
step S2 of FIG. 14 and the following steps. Thus, the phase .phi.C2
of the second cancel signal is controlled so as to minimize the
direct current component D2 of the demodulated second composite
signal. It will be understood that the steps S7 and S29 correspond
to the first-cancel-signal control portion 26, and the steps S35
and S59 correspond to the second-cancel-signal control portion 30,
while the steps S12 and S32 correspond to the
first-composite-signal-amplitude detecting portion 38.
[0130] The RFID-tag communication device 12 according to the first
embodiment described above may be modified to further include
compensating means for compensating the amplitudes of the first and
second cancel signals, in particular, for a variation in the phases
of the first signal combining portion 58 and the second signal
combining portion 66, and an influence of the gain G1 of the second
amplifying portion 60, for example. In a preferred modification,
predetermined compensating amounts are preferably stored in a
memory device of the DSP 16, and the amplitudes and phases of the
first and second cancel signals are determined so as to minimize
the amplitude of the leakage signal received by the RFID-tag
communication system 10, and the amplitude A1 of the first cancel
signal and the amplitude A2 of the second cancel signal are
compensated to further reduce the amplitude of the leakage signal.
For instance, the modification uses a control routine illustrated
in the flow chart of FIG. 20 in place of the control routine
illustrated in the flow charts of FIGS. 14-19. The control routine
of FIG. 20 is initiated with step S60 corresponding to the step S1
of FIG. 14, to perform an initial setting. Then, the control flow
goes to step S61 corresponding to the steps S2-S7, to determine the
amplitude A1 of the first cancel signal, and to step S62
corresponding to the steps S8-S29 of FIGS. 15 and 16, to determine
the phase .phi.C1 of the first cancel signal. Then, step S63 is
implemented to compensate the amplitude A1 of the first cancel
signal for a possible variation due to a change of the phase of the
first signal combining portion 58 and the gain G1 of the second
amplifying portion 60. In this case, the compensation of the
amplitude A1 is effected so as to minimize the amplitude Am1 of the
first composite signal. Step S64 corresponding to the steps S30-S34
of FIG. 17 is then implemented to determine the amplitude A2 of the
second cancel signal. The control flow then goes to step S75
corresponding to the steps S36-S59 of FIGS. 18 and 19, to determine
the phase .phi.C2 of the second cancel signal. Step S66 is then
implemented to compensate the amplitude A2 of the second cancel
signal for a possible variation due to a change of the phase of the
second signal combining portion 66 and the gain G2 of the second
signal combining portion 66, so as to minimize the direct current
component D2 of the demodulated second composite signal. In the
modification form of the first embodiment described above, the
steps S63 and S66 are implemented in addition to the steps
described above with respect to the first embodiment. It will be
understood that the steps S61-S63 correspond to the
first-cancel-signal control portion 26, while the steps S64-S66
correspond to the second-cancel-signal control portion 30.
[0131] The local-analog-oscillation-signal output portion 44 may be
arranged to effect frequency hopping of the local analog
oscillation signal. FIG. 21 is a view for explaining the frequency
hopping of the local analog oscillation signal. As shown in FIG.
21, the frequency of the local analog oscillation signal may be
hoped sequentially, for example, from fh1 to fh2, from fh2 to fh3,
and from fh3 to fh4, so that the frequency of the transmission
signal to be transmitted from the transmitter/receiver antenna 52
changes at a predetermined time interval, such that the frequency
is equal to f+fh1 at a point of time T1, f+fh2 at a point of time
T2, f+fh3 at a point of time T3, and f+fh4 at a point of time T4,
for example, wherein "f" represents the frequency of the
transmission signal which has been converted into the analog signal
by the transmission-signal D/A converting portion 42. In the
present arrangement, initial values of the amplitude A1 and phase
.phi.C1 of the first cancel signal, and the amplitude A2 and phase
.phi.C2 of the second cancel signal are stored for each of the
different frequency values to which the frequency of the local
analog oscillation signal is hopped, so that the initial values are
selected depending upon the present frequency of the local analog
oscillation signal during the frequency hopping. This arrangement
permits suppression of the leakage signal even where the frequency
of the local analog oscillation signal is hopped.
[0132] As described above, the RFID-tag communication device 12
includes: the first-cancel-signal output portion 24 operable to
generate the first cancel signal in the form of a digital signal
for suppressing from the received signal the leakage signal that is
a part of the transmission signal that is a part of the
transmission signal which is transmitted from the
transmitter/receiver antenna 52 and which is returned to and
received by the antenna 52; the first-cancel-signal control portion
26 operable to control the amplitude A1 and/or the phase .phi.C1 of
the first cancel signal generated by the first-cancel-signal output
portion 24; the first-cancel-signal D/A converting portion 54
operable to convert the first cancel signal generated by the
first-cancel-signal output portion 24, into an analog signal; and
the first signal combining portion 58 operable to combine together
the first cancel signal which has been converted into the analog
signal by the first-cancel-signal D/A converting portion 54, and
the received signal, to obtain the first composite signal.
Accordingly, the present RFID-tag communication device 12 does not
require a phase shifter for controlling the first cancel signal,
and permits easy control of the amplitude and/or phase of the first
cancel signal by digital signal processing. Namely, the present
RFID-tag communication device 12 is simple in construction and is
capable of suppressing the leakage signal that is a part of the
transmission signal which is transmitted from by the RFID-tag
communication device 12 and which is returned to and received by
the communication device 12.
[0133] The RFID-tag communication device 12 further includes: the
second-cancel-signal output portion 28 operable to generate the
second cancel signal in the form of a digital signal for
suppressing from the received signal the leakage signal received by
the communication device 12; the second-cancel-signal control
portion 30 operable to control the amplitude A2 and/or the phase
.phi.C2 of the second cancel signal generated by the
second-cancel-signal output portion 28; the second-cancel-signal
D/A converting portion 64 operable to convert the second cancel
signal generated by the second-cancel-signal output portion 28,
into an analog signal; and the second signal combining portion 66
operable to combine together the second cancel signal which has
been converted into the analog signal by the second-cancel-signal
D/A converting portion 64, and the received signal, to obtain the
second composite signal. Accordingly, the present RFID-tag
communication device 12 permits secondary suppression of the
leakage signal at the second signal combining portion, as well as
primary suppression of the leakage signal at the first signal
combining portion. Further, the present RFID-tag communication
device 12 does not require the phase shifter for controlling the
cancel signal, and permits easy control by digital signal
processing. It is noted that it is difficult to provide a phase
shifter practically operable to deal with a relatively low or
intermediate frequency. In this respect, the digital processing
according to the first embodiment is highly significant.
[0134] The first cancel signal and the second cancel signals have
respective different frequencies, and can therefore be easily be
controlled according to the control signals corresponding to the
frequencies of the first and second cancel signals.
[0135] The RFID-tag communication device 12 further includes the
second amplifying portion 60 the gain G1 of which is variable and
which is interposed between the first signal combining portion 58
and the second signal combining portion 66 and operable to amplify
the first composite signal generated by the first signal combining
portion 58. Accordingly, the first composite signal and the second
cancel signal can be suitably combined together to obtain the
second composite signal by the second signal combining portion 66.
Further, the high-frequency amplification of the first composite
signal by the second amplifying portion 60 improves the
signal-to-noise ratio, and permits demodulation of the reply
signals from the RFID tags 14, with a high degree of
sensitivity.
[0136] Further, the second signal combining portion 66 also
functions as an amplifying portion the gain G3 of which is variable
and which is operable to amplify the second composite signal, so
that the second composite signal can be detected with high
sensitivity, by analog-to-digital conversion of the second
composite signal or demodulation of the second composite
signal.
[0137] The RFID-tag communication device 12 further includes the
received-signal-amplitude detecting portion 36 operable to detect
the amplitude AR of the received signal, so that the
first-cancel-signal control portion 26 can control the amplitude A1
of the first cancel signal on the basis of the amplitude AR of the
received signal detected by the received-signal-amplitude detecting
portion 36, whereby the leakage signal contained in the received
signal can be effectively suppressed.
[0138] The RFID-tag communication device 12 further includes the
first-composite-signal-amplitude detecting portion 38 operable to
detect the amplitude AM1 of the first composite signal generated by
the first signal combining portion 58, so that the
second-cancel-signal control portion 26 can control the phase
.phi.C1 of the first cancel signal on the basis of the amplitude
AM1 of the first composite signal detected by the
first-composite-signal-amplitude detecting portion 38, whereby the
leakage signal contained in the received signal can be effectively
suppressed.
[0139] Further, the second-cancel-signal control portion 30 is
operable to control the amplitude AM2 of the second cancel signal
on the basis of the amplitude AM1 of the first composite signal
detected by the first-composite-signal-amplitude detecting portion
38, so that the leakage signal contained in the received signal can
be effectively suppressed.
[0140] The RFID-tag communication device 12 further includes the
direct-current-component detecting portion 34 operable to detect
the direct current component of the demodulated signal generated by
the demodulating portion 32 provided to demodulate the second
composite signal generated by the second signal combining portion
66, so that the second-cancel-signal control portion 30 can control
the phase .phi.C2 of the second cancel signal on the basis of the
direct current component of the demodulated signal detected by the
direct-current-component detecting portion 34, whereby the leakage
signal contained in the received signal can be effectively
suppressed.
[0141] The RFID-tag communication device 12 further includes: the
digital-transmission-signal output portion 20 operable to generate
the transmission signal in the form of a digital signal; the
transmission-signal D/A converting portion 42 operable to convert
the transmission signal generated by the
digital-transmission-signal output portion 20, into an analog
signal; the first-composite-signal A/D converting portion 70 which
is interposed between the first signal combining portion 58 and the
first-composite-signal-amplitude detecting portion 38 and which is
operable to convert the first composite signal generated by the
first signal combining portion 58, into a digital signal; the
second-composite-signal A/D converting portion 72 which is
interposed between the second signal combining portion 66 and the
demodulating portion 32 and which is operable to convert the second
composite signal generated by the second signal combining portion
66, into a digital signal; and the received-signal A/D converting
portion 76 operable to convert the received signal into a digital
signal. The above-described first-cancel-signal D/A converting
portion 54, second-cancel-signal D/A converting portion 64,
transmission-signal D/A converting portion 42,
first-composite-signal A/D converting portion 70,
second-composite-signal-A/D converting portion 72 and
received-signal A/D converting portion 76 use the common clock
signal generated by the clock-signal output portion 78, to prevent
a difference in the reference phase between the transmission signal
and the received signal, thereby permitting effective suppression
of the leakage signal contained in the received signal. It is noted
that the demodulation of the received signal the frequency of which
has been reduced by the first down-converter 62 to an intermediate
frequency has a risk of considerable generation of a relatively low
frequency component upon the demodulation due to a difference
between the frequency of the clock signal of the A/D converting
portions and the intermediate frequency. However, this risk can be
prevented by using the common clock signal for the
digital-to-analog conversion and the analog-to-digital
conversion.
[0142] The RFID-tag communication device 12 further includes: the
local-oscillation-signal output portion 44 operable to generate the
predetermined local oscillation signal; the first up-converter 46
operable to increase the frequency of the transmission signal which
has been converted into an analog signal by the transmission-signal
D/A converting portion 43, by an amount corresponding to the
frequency of the local oscillation signal generated by the
local-oscillation-signal output portion 44; and the first
down-converter 62 operable to reduce the frequency of the first
composite signal generated by the first signal combining portion
58, by an amount corresponding to the frequency of the local
oscillation signal generated by the local-oscillation-signal output
portion 44. Accordingly, the analog-to-digital conversion of the
first composite signal and the digital-to-analog conversion of the
transmission signal can be effected by a simple converter
arrangement using relatively inexpensive A/D and D/A
converters.
[0143] The RFID-tag communication device 12 further includes the
second down-converter 74 operable to reduce the frequency of the
received signal, by an amount corresponding to the frequency of the
local oscillation signal generated by the local-oscillation-signal
output portion 44. Accordingly, the analog-to-digital conversion of
the received signal can be effected by using a simple converter
arrangement using a relatively inexpensive A/D converter.
[0144] Further, the digital-transmission-signal output portion 20
is arranged to generate the transmission signal on the basis of the
predetermined sampling values which correspond to the respective
different phases at the predetermined sampling points and which are
represented by the function table stored in the function table 40.
Accordingly, the digital-transmission-signal output portion 20 can
generate the digital transmission signal, with a relatively simple
arrangement.
[0145] Further, the first-cancel-signal output portion 24 is
arranged to generate the first cancel signal on the basis of the
function table stored in the function table 40, and the
first-cancel-signal control portion 26 is arranged to control the
phase .phi.C1 of the first cancel signal, by changing the positions
of the function table from which the function values are read out.
Accordingly, the first-cancel-signal output portion 24 can generate
the digital first cancel signal, with a relatively simple
arrangement, and the phase .phi.C1 of the first cancel signal can
be easily controlled.
[0146] Further, the first-cancel-signal control portion 26 is
arranged to control the amplitude A1 of the first cancel signal, by
multiplying the digital signal generated on the basis of the
function table stored in the function table 40, by a suitable
control value. Thus, the amplitude A1 of the first cancel signal
can be easily controlled.
[0147] Further, the second-cancel-signal output portion 28 is
arranged to generate the second cancel signal on the basis of the
function table stored in the function table 40, and the
second-cancel-signal control portion 30 is arranged to control the
phase .phi.C2 of the second cancel signal, by changing the
positions of the function table from which the function values are
read out. Accordingly, the second-cancel-signal output portion 26
can generate the second cancel signal, with a relatively simple
arrangement, and the phase .phi.C2 of the second cancel signal can
be easily controlled.
[0148] Further, the second-cancel-signal control portion 30 is
arranged to control the amplitude A2 of the second cancel signal,
by multiplying the digital signal generated on the basis of the
function table in the function table 40, by a suitable control
value. Thus, the amplitude A2 of the second cancel signal can be
easily controlled.
[0149] Further, the first-cancel-signal output portion 26 is
arranged to control the amplitude A1 and the phase .phi.C1 of the
first cancel signal on the basis of the received signal or the
output of the second down-converter 74, and to control the phase
.phi.C1 of the first cancel signal on the basis of the second
composite signal generated by the second signal combining portion
66. This arrangement permits effective suppression of the leakage
signal contained in the received signal. Where the
first-cancel-signal control portion is arranged to set the initial
values of the amplitude A1 and phase .phi.C1 of the first cancel
signal on the basis of the received signal or the first composite
signal, the time required for subsequent control of the phase
.phi.C1 of the first cancel signal can be reduced.
[0150] The local-oscillation-signal output portion 44 is arranged
to effect the frequency hopping of the local oscillation signal.
This arrangement is effective to prevent the
local-oscillation-signal output portion 44 from disturbing or being
disturbed by an operation of radio communication not associated
with the ratio communication with the RFID tag 14.
[0151] There will be described in detail second through sixth
embodiments of this invention by reference to the accompanying
drawings. The same reference signs as used in the first embodiment
will be used to identify the same elements, which will not be
described.
Embodiment 2
[0152] Referring to FIG. 22 showing an electrical arrangement of an
RFID-tag communication device 92 constructed according to the
second embodiment of this invention, the transmitter/receiver
circuit 18 of this RFID-tag communication device 92 includes an
RSSI (received signal strength indicator) 94 and an
amplitude-signal A/D converting portion 96. The RSSI 94 is arranged
to detect the amplitude AR of the received signal received from the
transmission/reception separator 50, and the amplitude-signal A/D
converting portion 96 is arranged to convert an output of the RSSI
94 indicative of the amplitude AR of the received signal, into a
digital signal to be applied to the first-cancel-signal control
portion 26. In the present second embodiment, the amplitude AR of
the received signal received through the antenna 52 is directly
detected by the RSSI 94, and the signal indicative of the detected
amplitude AR is converted by the amplitude-signal A/D converting
portion 96 into the digital signal to be applied to the
first-cancel-signal control portion 26, so that the
transmitter/receiver circuit 18 is advantageously simplified in
construction.
Embodiment 3
[0153] Referring to FIG. 23 showing an electrical arrangement of an
RFID-tag communication device 98 constructed according to the third
embodiment of this invention, the present communication device 98
does not include the function table 40, and includes a
digital-transmission-signal output portion 100, a
first-cancel-signal output portion 102, and a second-cancel-signal
output portion 104 in place of the digital-transmission-signal
output portion 20, first-cancel-signal output portion 24 and
second-cancel-signal output portion 26. The output portions 100,
102, 104 are operable independently of each other, to generate
digital signals in the form of sine-wave or cosine-wave signals,
without using the function table 40 provided in the first
embodiment. Reference is made to FIG. 24 showing an arrangement of
each of the digital-transmission-signal output portion 100,
first-cancel-signal output portion 102 and second-cancel-signal
output portion 104. Each of these output portions 100, 102, 104
includes an integrator 106 and an arithmetic unit 108. The
integrator 106 is arranged to calculate a phase .theta.
(=.omega..sub.0nT) by incrementing an initial phase .phi. by a
predetermined value .DELTA..theta. (e.g., .pi./2). The arithmetic
unit 108 is arranged to calculate a cosine wave cos .theta.
corresponding to the phase .phi. calculated by the integrator 106.
To change the phase .theta. by a desired amount.+-..DELTA..phi.,
the initial phase is changed by the desired amount.+-..DELTA..phi.,
by a switching operation. The digital-transmission-signal output
portion 100, first-cancel-signal output portion 102 and
second-cancel-signal output portion 104 perform arithmetic
operations to calculate the desired amount.+-..DELTA..phi., from
time to time, and multiply a cosine wave cos .theta. calculated by
the arithmetic unit 108, by a control value depending upon the
phase .theta. calculated by the integrator 106, for thereby
amplifying the cosine wave cos .theta., to generate a desired
digital signal in the form of the cosine wave. The present third
embodiment does not require the function table 40, and does not
require the frequency of the clock signal generated by the
clock-signal output portion 78 to be set four times the frequency
of the first composite signal (intermediate frequency signal) the
frequency of which has been reduced by the first down converter 62,
or a multiple of the frequency four times that of this intermediate
frequency signal, in order to simplify the function table stored in
the function table 40. Thus, the present third embodiment permits
the first and second cancel signals having the desired phase and
amplitude. The amplitude and phase of the received signal may be
detected without the provision of the received-signal-amplitude
detecting portion 36. In this case, the first-cancel-signal output
portion 102 may be modified to generate a first cancel signal
having the detected amplitude of the received signal and a phase
opposite to the detected phase of the received signal. The phase of
the first cancel signal is subsequently adjusted so as to minimize
the output of the first-composite-signal-amplitude detecting
portion 38. This modification is effective to maximize the control
speed of the first cancel signal.
Embodiment 4
[0154] Referring to FIG. 25 showing an electrical arrangement of an
RFID-tag communication device 110 constructed according to the
fourth embodiment of this invention, this communication device 110
includes a third-cancel-signal output portion 112, a
third-cancel-signal D/A converting portion 114, a third
down-converter 116 and a third signal combining portion 118. The
third-cancel-signal output portion 112 is arranged to generate a
third cancel signal in the form of a digital signal on the basis of
the received signal. The third-cancel-signal D/A converting portion
114 is arranged to convert the third cancel signal generated by the
third-cancel-signal output portion 112, into an analog signal. The
third down-converter 116 is arranged to reduce the frequency of the
analog third cancel signal generated by the third-cancel-signal D/A
converting portion 114, by an amount corresponding to the frequency
of the local oscillation signal generated by the
local-oscillation-signal output portion 44. The third signal
combining portion 118 is arranged to combine together the third
cancel signal the frequency of which has been reduced by the third
down converter 116, and the received signal received from the
transmission/reception separator 50. In the present fourth
embodiment, the leakage signal contained in the received signal is
suppressed by the third signal combining portion 118, before the
frequency of the received signal is reduced by the second
down-converter 74, so that the reply signal from each RFID tag 14
can be detected with high sensitivity, even where the received
signal contains a comparatively large amount of the leakage signal
(that is a part of the transmission signal transmitted from and
returned to the communication device), or contains noise signals
mixed therein due to reflection of the transmitted transmission
signal by any structural body located near the communication
device. In addition, the present RFID-tag communication device 110
permits the second down-converter 62 to have a relatively low upper
limit of the input voltage, thereby making it possible to reduce
the amount of noises remaining in the output of the second
down-converter 74, whereby the sensitivity of the RFID-tag
communication device 110 to the reply signal transmitted from the
RFID tag 14 can be improved. Since the output of the third
down-converter 116 is not applied to the demodulator 32, the third
down-converter 116 does not have an adverse effect on the
sensitivity of the communication device 10 to the reply signal even
where the third down-converter 116 has a relatively high lower
limit of the input voltage.
[0155] In the preceding embodiments, the first-cancel-signal output
portion 24, 102, first-cancel-signal control portion 26,
second-cancel-signal output portion 28, 104, second-cancel-signal
control portion 30, etc. are functional elements incorporated in
the DSP 16, those portions may be control devices separate from the
DSP 16.
[0156] Although the second signal combining portion 66 in the
preceding embodiments also functions as an amplifier to amplify the
second composite signal, a second-composite-signal amplifying
portion may be interposed between the second signal combining
portion 66 and the second-composite-signal A/D converting portion
72. Further, the first signal combining portion 58 may be arranged
to also function as an amplifier to amplify the first composite
signal. In this case, the second amplifying portion 60 is not
required.
[0157] In the preceding embodiments, the RFID-tag communication
device 12, 92, 98, 110 is provided with the transmitter/receiver
antenna 52 used to transmit the carrier wave F.sub.c1 toward the
RFID tags 14 and receive the reflected waves F.sub.rf transmitted
from the RFID tags 14. However, the RFID-tag communication device
may use a transmitter antenna through which the carrier wave
F.sub.c1 is transmitted toward the RFID tags 14, and a receiver
antenna through which the reflected waves F.sub.rf transmitted from
the RFID tags 14 are received.
[0158] In the preceding embodiments, the RFID-tag communication
device 12, 92, 98, 110 is provided with the first down-converter 62
operable to reduce the frequency of the first composite signal to
generate an intermediate frequency signal. However, the provision
of the first down-converter 62 to reduce the frequency of the first
composite signal is not essential. The suppression of the first
down-converter 62 simplifies the arrangement of the
transmitter/receiver circuit 18.
[0159] While the RFID-tag communication device 12, 92, 98, 110 in
the preceding embodiments is used as an interrogator in the
communication system 10, the RFID-tag communication device may be
used as an RFID-tag producing device for writing desired
information on the RFID tags 14, or an RFID-tag reader/writer for
reading or writing information from or on the RFID tags 14.
Embodiment 5
[0160] Referring to FIG. 26 schematically showing an overall
arrangement of a communication system S constructed according to
the fifth embodiment of this invention, the communication system S
is an RFID (radio-frequency identification) communication system
including an RFID-tag communication device in the form of an
interrogator 1 and a transponder in the form of an RFID tag T. The
communication system S may include a plurality of interrogators 1,
and a plurality of RFID tags T. The RFID tag T has an RFID-circuit
element T.sub.0 including the antenna portion 80, the
modulating/demodulating portion 82 and the digital circuit portion
(IC-circuit portion) 84, like the RFID tags 14 in the first
embodiment described above.
[0161] The interrogator 1 arranged to effect radio communication
with the antenna portion 80 of the RFID-circuit element T.sub.0
consists of a transmitter/receiver antenna 2, a DSP (digital signal
processor) 3 and a transmitter/receiver circuit 4. The DSP 3 is
arranged to effect digital signal processing operations to effect
at least one of information writing and reading on and from the
RFID-circuit element T.sub.0 through the antenna 2, by generating a
digital transmission signal in the form of an interrogating wave
F.sub.c, and/or demodulating a reply signal in the form of a
reflected wave F.sub.r transmitted from the RFID-circuit element
T.sub.0. The transmitter/receiver circuit 34 is arranged to convert
the transmission signal generated by the DSP 3 into an analog
signal, transmit the analog transmission signal as the
interrogating wave F.sub.c through the antenna 2, receive the
reflected wave F.sub.r from the RFID-circuit element T.sub.0,
convert the received reflected wave F.sub.r into a digital signal,
and apply the digital signal to the DSP 3.
[0162] The block diagram of FIG. 27 shows a functional arrangement
of the interrogator 1, which consists of the antenna 2, DSP3 and
transmitter/receiver circuit 4, as described above.
[0163] The DSP3 is constituted by a so-called microcomputer system
incorporating a CPU, a ROM and a RAM and operable to perform signal
processing operations according to control programs stored in the
ROM, while utilizing a temporary data storage function of the RAM.
The DSP3 includes a function table 140, a
digital-transmission-signal output portion 120, a 90.degree. phase
shifting portion (phase converting portion) 141, a modulating
portion 122, a first-cancel-control signal output portion 124, a
first-cancel-control-signal control portion 126, a
second-cancel-control-signal output portion 128, a
second-cancel-control-signal control portion 130, a demodulating
portion 132, a direct-current-component detecting portion 134, a
received-signal-amplitude detecting portion 136, and a
first-composite-signal-amplitude detecting portion 138. The
function table is provided to store sampling values corresponding
to respective different phases at predetermined sampling points.
The digital-transmission-signal output portion 120 is arranged to
generate the digital transmission signal in the form of a carrier
wave (sine-wave signal) to be transmitted to the RFID-circuit
element T.sub.0, on the basis of sampling values of a function
table stored in the function table 140. The 90.degree. phase
shifting portion 141 is arranged to generate, on the basis of the
sampling values stored in the function table 140, a digital signal
(sine-wave signal) which is delayed with respect to the digital
signal generated by the digital-transmission-signal output portion
120, by a time corresponding to one sampling value, namely, a
digital transmission signal the phase of which is different by
90.degree. from that of the digital transmission signal generated
by the digital-transmission-signal output portion 120. The
modulating portion 122 is arranged to modulate the above-indicated
carrier wave on the basis of a suitable command signal, for thereby
generating an access signal. The first-cancel-control-signal output
portion 124 is arranged to generate a first cancel control signal
for generating a first cancel signal (first offset signal) to
effect primary suppression or cancellation (offsetting) of an
unnecessary or interference wave that is a part of the digital
transmission signal which is transmitted from the antenna 2 toward
the RFID-circuit element T.sub.0 and returned to the
transmission/receiver circuit 4. The first-cancel-control-signal
control portion 126 is arranged to control the amplitude and phase
of the first cancel control signal generated by the
first-cancel-control-signal output portion 124. The
second-cancel-control-signal output portion 128 is arranged to
generate a second cancel control signal for generating a second
cancel signal (second offset signal) to effect secondary
suppression or cancellation of the above-indicated unnecessary
wave. The second-cancel-control-signal control portion 130 is
arranged to control the amplitude and phase of the second cancel
control signal generated by the second-cancel-control-signal output
portion 128. The demodulating portion 132 is arranged to demodulate
a received signal received through the antenna 2. The
direct-current-component detecting portion 134 is arranged to
detect a direct-current component (DC component) of the demodulated
signal generated by the demodulating portion 132. The
received-signal-amplitude detecting portion 136 is arranged to
detect the amplitude of the above-indicated received signal. The
first-composite-signal-amplitude detecting portion 138 is arranged
to detect the amplitude of a first composite signal generated by a
first signal combining portion 158 (which will be described).
[0164] The transmitter/receiver circuit 4 includes a high-speed
first transmission-signal D/A converting portion 142, a high-speed
second transmission-signal D/A converting portion 143, a
local-oscillation-signal output portion 144, a first up-converter
146, a first amplifying portion 148, a transmission/reception
separator 150, low-speed first-cancel-control-signal D/A converting
portions 153, 154, a first-composite-sine-wave-signal generating
circuit 180, a second up-converter 156, the above-indicated first
signal combining portion (synthesizer) 158, a second amplifying
portion 160, a first down-converter 162, low-speed
second-cancel-control-signal D/A converting portions 163, 164, a
second-composite-sine-wave-signal generating circuit 190, a second
signal combining portion 166, a third amplifying portion 168, a
first-composite-signal A/D converting portion 170, a
second-composite-signal A/D converting portion 172, a second
down-converter 174, a received-signal A/D converting portion 176,
and a clock-signal output portion 178. The high-speed first
transmission-signal D/A converting portion 142 is arranged to
convert the digital transmission signal generated by the
digital-transmission-signal output portion 120, into an analog
signal, for generating a first sine-wave signal. The high-speed
second transmission-signal D/A converting portion 143 is arranged
to convert the digital transmission signal generated by the
90.degree. phase shifting portion 141 into an analog signal, for
generating a second sine-wave signal the phase of which is
different by 90.degree. from that of the first sine-wave signal
indicated above. The local-oscillation-signal output portion 144 is
arranged to generate a suitable local oscillation signal. The first
up-converter 146 is arranged to increase the frequency of the
analog transmission signal received from the above-described first
transmission-signal D/A converting portion 142, by an amount
corresponding to the frequency of the local oscillation signal
generated by the local-oscillation-signal output portion 144. The
first amplifying portion 148 is arranged to amplify the
transmission signal generated by the first up-converter 146. The
transmission/reception separator 150 is arranged to apply the
transmission signal generated by the first amplifying portion 148,
to the transmitter/receiver antenna 2, and to apply the reply
signal (the received signal containing a leakage signal that is a
part of the transmission signal transmitted from the communication
device and returned to the communication device) received from the
above-described RFID-circuit element T.sub.0 through the
transmitter/receiver antenna 2, to the first signal combining
portion 158 and the second down-converter 174. The low-speed
first-cancel-control-signal D/A converting portions 153, 154 are
arranged to convert the first cancel control signal generated by
the first-cancel-control-signal output portion 124, into amplitude
signals. The first-composite-sine-wave-signal generating circuit
180 is arranged to receive the first and second sine-wave signals
from the first transmission-signal D/A converting portion 142 and
the second transmission-signal D/A converting portion 143, and
combine together the received first and second sine-wave signals to
obtain a composite sine-wave signal (first cancel signal for the
primary suppression or cancellation of the unnecessary wave) having
an amplitude and a phase that are different from those of the first
and second sine-wave signals, on the basis of the amplitude control
signals received from the first-cancel-control-signal D/A
converting portions 153, 154 and polarity control signals received
from the first-cancel-control-signal output portion 124, while the
amplitudes of the first and second sine-wave signals
are-controlled. The second up-converter 156 is arranged to increase
the frequency of the first cancel signal generated by the
first-composite-sine-wave-signal generating portion 180, by an
amount corresponding to the frequency of the local oscillation
signal generated by the above-described local-oscillation-signal
output portion 144. The first signal combining portion
(synthesizer) 158 is arranged to combine together the first cancel
signal generated by the second up converter 156, and the
above-described received signal received from the
transmitter/receiver antenna 2 through the transmission/reception
separator 150, to obtain a first composite signal. The second
amplifying portion 160 is arranged to amplify the first composite
signal generated by the first signal combining portion 158, for
increasing its amplitude. The first down-converter 162 is arranged
to reduce the frequency of the first composite signal generated by
the second amplifying portion 160, by an amount corresponding to
the frequency of the local oscillation signal generated by the
above-described local-oscillation-signal output portion 144. The
low-speed second-cancel-control-signal D/A converting portion 163,
164 are arranged to convert the second cancel control signal
generated by the above-described second-cancel-control signal
output portion 128, into amplitude signals. The
second-composite-sine-wave-signal generating circuit 190 is
arranged to receive the first and second sine-wave signals from the
first transmission-signal D/A converting portion 142 and the second
transmission-signal D/A converting portion 143, combine together
the received first and second sine-wave signals to synthesize a
composite sine-wave signal (second cancel signal for the secondary
suppression or cancellation of the unnecessary wave) having an
amplitude and a phase that are different from those of the first
and second sine-wave signals, on the basis of the amplitude control
signals received from the second-cancel-control-signal D/A
converting portions 163, 164 and polarity control signals received
from the first-cancel-control-signal output portion 128, while the
amplitudes of the first and second sine-wave signals are
controlled. The second signal combining portion 166 is arranged to
combine together (and amplify, when needed) the second cancel
signal generated by the second-composite-sine-wave-signal
generating portion 190 and the first composite signal generated by
the first down-converter 162, to obtain a second composite signal.
The third amplifying portion 168 is arranged to amplify the first
composite signal generated by the first down-converter 162, for
increasing its amplitude. The first-composite-signal A/D converting
portion 170 is arranged to convert the first composite signal
received from the third amplifying portion 168, into a digital
signal, and to apply the digital first composite signal to the
above-described first-composite-signal-amplitude detecting portion
138. The second-composite-signal A/D converting portion 172 is
arranged to convert the second composite signal received from the
second signal combining portion 166, into a digital signal, and to
apply the digital second composite signal to the above-described
demodulating portion 132. The second down-converter 174 is arranged
to reduce the frequency of the above-described received signal
received through the transmission/reception separator 150, by an
amount corresponding to the frequency of the local oscillation
signal generated by the local-oscillation-signal output portion
144. The received-signal A/D converting portion 176 is arranged to
convert the received signal received from the second down-converter
174, into a digital signal, and to apply the digital received
signal to the above-described received-signal-amplitude detecting
portion 136. The clock-signal output portion 178 is arranged to
generate a suitable clock signal.
[0165] The clock-signal output portion 178 applies the clock signal
to the above-described transmission-signal D/A converting portions
142, 143, and applies the same clock signal to the above-described
first-composite-signal A/D converting portion 170,
second-composite-signal A/D converting portion 172, and
received-signal A/D converting portion 176. The above-described
received-signal A/D converting portion 176 uses a converter having
a smaller number of bits, than a converter used by the
first-composite-signal A/D converting portion 170, etc. This
converter used by the received-signal A/D converting portion 176
has an advantage that a component of the received signal which
relates to the modulation by the RFID-circuit element T.sub.0 can
be ignored. It is noted that the local-oscillation-signal output
portion 144 uses an oscillator operable to generate a frequency in
the neighborhood of 900 MHz or 2.4 GHz. The transmission/reception
separator 150 generally uses a circulator or a directional
coupler.
[0166] A basic operation of the RFID communication system S will be
described by reference to FIGS. 28-32.
[0167] As shown in FIG. 27, the digital-transmission-signal output
portion 120 of the DSP 3 of the interrogator 1 in the present RFID
communication system S generates the digital transmission signal on
the basis of the function table stored in the function table 140.
The generated digital transmission signal is converted by the first
transmission-signal D/A converting portion 142 into an analog
signal (sine-wave signal).
[0168] The frequency of the analog transmission signal generated by
the first transmission-signal D/A converting portion 142 is
increased by the first up-converter 146 by an amount corresponding
to the frequency of the local oscillation signal generated by the
local-oscillation-signal output portion 144, and the analog
transmission signal generated by the first up-converter 146 is
amplified by the first amplifying portion 148 and modulated
according to a signal received from the modulating portion 122. The
modulated transmission signal generated by the first amplifying
portion 148 is applied to the antenna 2 through the
transmission/reception separator 150, and transmitted as the
interrogating wave F.sub.c from the antenna 2 toward the
RFID-circuit element T.sub.0.
[0169] The interrogating wave F.sub.c transmitted from the antenna
2 and received by the antenna 80 of the RFID-circuit element
T.sub.0 as shown in FIG. 26 is modulated by the
modulating/demodulating portion 82. A portion of the interrogating
wave F.sub.c is rectified by a rectifying portion of the
RFID-circuit portion T.sub.0, and is used as an energy source
(power source) of the RFID-circuit element T.sub.0. With an
electric energy of this energy source, the control portion 86 of
the Digital-circuit portion 84 generates the reply signal on the
basis of information signals stored in a memory of the RFID-circuit
element T.sub.0, and the modulating/demodulating portion 82
modulates the interrogating wave F.sub.c on the basis of the
generated reply signal. The modulated interrogating wave F.sub.c is
transmitted as the reflected wave F.sub.r from the antenna portion
80 toward the interrogator 1.
[0170] The reflected wave F.sub.r transmitted from the RFID-circuit
element T.sub.0 is received by the antenna 2 of the interrogator 1,
and applied as the received signal to the first signal combining
portion 158 and the second down-converter 174 through the
transmission/reception separator 150. At this time, a leakage
signal that is a part of the transmission signal which is
transmitted from the RFID-tag communication device and returned to
the communication device through the transmission/reception
separator 150 may be applied to the first signal combining portion
158 and second down-converter 174, together with the received
signal.
[0171] The frequency of the received signal received by the second
down-converter 174 is reduced by the amount corresponding to the
frequency of the local oscillation signal generated by the
local-oscillation-signal output portion 144. The received signal
the frequency of which has been reduced by the second
down-converter 74 is converted by the received-signal A/D
converting portion 176, into a digital signal which is applied to
the received-signal-amplitude detecting portion 136, so that the
amplitude of the received signal is detected by the
received-signal-amplitude detecting portion 136. An output of the
received-signal-amplitude detecting portion 136 is applied to the
first-cancel-control-signal control portion 126.
[0172] The first-cancel-control-signal control portion 126
determines the phase and the amplitude of the first cancel control
signal for the primary suppression or cancellation (offsetting) of
the leakage signal, on the basis of the amplitude of the received
signal received from the received-signal-amplitude detecting
portion 136 and the amplitude of the received signal received from
the first-composite-signal-amplitude detecting portion 138, as
shown in FIG. 27. The output of the first-cancel-control-signal
control portion 126 is applied to the first-cancel-control-signal
output portion 124. The first-cancel-signal output portion 124
generates the first cancel control signal in the form of a digital
signal having the phase and amplitude determined by the
first-cancel-control-signal control portion 126.
[0173] The 90.degree. phase shifting portion 141 generates a
transmission signal in the form of a digital signal, on the basis
of the sampling values of the function table received by the
digital-transmission-signal output portion 120. This digital
transmission signal is converted by the second transmission-signal
D/A converting potion 143 into an analog signal (second sine-wave
signal). The second sine-wave signal generated by the second
transmission-signal D/A converting portion 143 and the first
sine-wave signal generated by the first transmission-signal D/A
converting portion 142 are applied to the
first-composite-sine-wave-signal generating circuit 180 and the
second composite-sine-wave-signal generating circuit 190.
[0174] The first-composite-sine-wave-signal generating circuit 180
is provided to combine together the received first and second
sine-wave signals to synthesize the composite sine-wave signal
(first cancel signal for the primary suppression or cancellation of
the leakage signal), on the basis of the first cancel control
signal received from the first-cancel-control-signal output portion
124, more precisely, polarity control signals Vswc1 and Vsws1
received from the first-cancel-control-signal output portion 124,
and amplitude control signals Vatc1 and Vats1 received from the
first-cancel-control-signal D/A converting portions 153, 154, while
the amplitudes of the first and second sine-wave signals are
controlled.
[0175] The amplitude of the first cancel signal generated by the
first-composite-sine-wave-signal generating circuit 180 is
increased by the second up-converter 156, by an amount
corresponding to the frequency of the local oscillation signal
generated by the local-oscillation-signal output portion 144. The
first cancel signal generated by the second up-converter 156 and
the received signal received through the transmission/reception
separator 150 are combined together by the first signal combining
portion 158 to obtain the first composite signal from which the
leakage signal received by the RFID-tag communication device 12 is
totally or partially suppressed.
[0176] The amplitude of the first composite signal generated by the
first signal combining portion 158 is amplified by the second
amplifying portion 160, by a predetermined gain. Where the
amplitude and the phase of the first cancel signal are suitably
determined, the intensity of the first cancel signal is relatively
low, and the intensity of the input to the first down-converter 162
is accordingly low, so that the gain of the second amplifying
portion 160 is accordingly increased.
[0177] The frequency of the first composite signal generated by the
second amplifying portion 160 is reduced by the first
down-converter 162 by the amount corresponding to the frequency of
the local oscillation signal generated by the local-signal output
portion 144, and the first composite signal the frequency of which
has been reduced is applied to the second signal combining portion
166 and the third amplifying portion 168. The gain of the third
amplifying portion 168 is set at a predetermined initial value. The
frequency of the clock signal generated by the clock-signal output
portion 178 is preferably four times the frequency of the first
composite signal (intermediate frequency signal) the frequency of
which has been reduced, or a multiple of the frequency four times
that of this intermediate frequency signal.
[0178] The amplitude of the first composite signal applied to the
third amplifying portion 168 is amplified by a predetermined gain.
The amplitude of the first composite signal generated by the first
down-converter 162 decreases as the suppression or reduction of the
leakage signal progresses. Accordingly, the gain of the third
amplifying portion 168 is preferably increased as the suppression
of the leakage signal progresses.
[0179] The first composite signal generated by the third amplifying
portion 168 is converted by the first-composite-signal A/D
converting portion 170, into the digital signal, and the digital
first composite signal is applied to the
first-composite-signal-amplitude detecting portion 138, so that the
amplitude of the first composite signal is detected by the
first-composite-signal-amplitude detecting portion 138. The output
of the first-composite-signal-amplitude detecting portion 138 is
applied to the first-cancel-control-signal control portion 126 and
the second-cancel-control-signal control portion 130. The phase and
amplitude of the first cancel control signal are controlled by the
first-cancel-control-signal control portion 126, on the basis of
the output of the received-signal-amplitude detecting portion 136
and the output of the first-composite-signal-amplitude detecting
portion 138, such that the amplitude of the first cancel signal
generated by the first-composite-sine-wave-signal generating
circuit 180 and converted by the first up-converter 156 is equal to
the amplitude of the received signal received by the first signal
combining portion 158, and such that the phases of those first
cancel signal and received signal are reversed with respect to each
other. Namely, the amplitude of the first cancel signal is
determined by the output of the received-signal-amplitude detecting
portion 136, and the phase of the first cancel signal is determined
on the basis of the output of the first-composite-signal-amplitude
detecting portion 138, so as to minimize the output of the
first-composite-signal-amplitude detecting portion 138.
[0180] In the meantime, the phase and the amplitude of the second
cancel control signal for the secondary suppression or cancellation
(offsetting) of the leakage signal are determined by the
second-cancel-control-signal control portion 130, on the basis of
the output of the first-composite-signal-amplitude detecting
portion 138 (indicative of the amplitude of the received signal
subjected to the primary suppression of the leakage signal) and the
second composite signal generated by the second-composite-signal
A/D converting portion 172, as shown in FIG. 27. The output of the
second-cancel-control-signal control portion 130 is applied to the
second-cancel-control-signal output portion 128, which generates
the second cancel control signal for the secondary suppression of
the leakage signal, in the form of a digital signal having the
phase and amplitude determined by the second-cancel-control signal
control portion 130.
[0181] Like the first:composite-sine-wave-signal generating circuit
180, the second-composite-sine-wave-signal generating circuit 190
is provided to combine together the received first and second
sine-wave signals to synthesize the composite sine-wave signal
(second cancel signal for the secondary suppression or cancellation
f the leakage signal), on the basis of the second cancel control
signal received from the second-cancel-control-signal output
portion 128, more precisely, polarity control signals Vswc2 and
Vsws2 received from the second-cancel-control-signal output portion
130, and amplitude control signals Vatc2 and Vats2 received from
the second-cancel-control-signal D/A converting portions 163, 164,
while the amplitudes of the first and second sine-wave signals are
controlled.
[0182] The second cancel signal generated by the
second-composite-sine-wave-signal generating circuit 190 and the
first composite signal the frequency of which has been reduced by
the first down-converter 162 are combined together by the second
signal combining portion 166 to obtain the second composite signal
from which the leakage signal has been totally or partially
suppressed.
[0183] The second composite signal generated by the second signal
combining portion 166 is converted into the digital signal by the
second-composite-signal A/D converting portion 172, and demodulated
by the modulating portion 132, whereby the reply signal (reflected
wave signal F.sub.r) received from the RFID-circuit element T.sub.0
is read. The second composite signal demodulated by the
demodulating portion 132 is applied to the direct-current-component
detecting portion 134, so that the direct current component of the
demodulated signal is detected by the direct-current-component
detecting portion 134. An output of the detecting portion 134 is
received by the second-cancel-control-signal control portion 130.
The phase and amplitude of the second cancel control signal are
controlled by the second-cancel-control-signal control portion 130,
on the basis of the amplitude of the direct current component which
is detected by the direct-current-component detecting portion 134
and which indicates the leakage signal, and the output of the
first-composite-signal-amplitude detecting portion 138, such that
the amplitude of the second cancel signal generated by the
second-composite-sine-wave-signal generating circuit 190 is equal
to the amplitude of the first composite signal converted by the
first down-converter 162 and received by the second signal
combining portion 166, and such that the phases of those second
cancel signal and first composite signal are reversed with respect
to each other. Namely, the amplitude of the second cancel signal is
determined by the output of the first-composite-signal-amplitude
detecting portion 138, and the phase of the second cancel signal is
determined so as to minimize the amplitude of the direct current
component detected by the direct-current-component detecting
portion 134.
[0184] In the RFID-tag communication system S of FIG. 27 the basic
operation of which has been described above, the leakage signal
that is a part of the transmission signal which is transmitted from
the interrogator 1 toward the RFID-circuit element T.sub.0 and
returned to the interrogator 1 can be totally or partially
suppressed from the received signal received through the antenna 2.
Thus, the communication system S is capable of radio communication
between the interrogator 1 and the RFID-circuit element T.sub.0,
with a high degree of sensitivity.
[0185] In the RFID-tag communication system S of the present fifth
embodiment constructed and operating as described above, the
first-composite-sine-wave-signal generating circuit 180 and the
second-composite-sine-wave-signal generating circuit 190 are
provided to combine together a cosine-wave signal (first sine-wave
signal) and a sine-wave signal (second sine-wave signal) which have
respective different amplitudes and respective phases having a
phase difference of 90.degree., to synthesize composite sine-wave
signals. Thus, the present RFID-tag communication system S has a
simple arrangement for changing the phases of the composite
sine-wave signals as desired.
[0186] FIGS. 28-32 are vectorial views for explaining the principle
of the fifth embodiment of this invention. The vectorial view of
FIG. 28 shows a composite sine-wave signal having a phase .theta.
and an amplitude A. This composite sine-wave signal is synthesized
by combining together a first sine-wave signal (cosine-wave signal)
having an amplitude A cos .theta., and a second sine-wave signal
which has a phase different 90.degree. from the phase of the first
sine-wave signal and which has an amplitude A sin .theta., namely,
by adding vectors of the first and second sine-wave signals. By
controlling the amplitudes of the first and second sine-wave
signals, a composite sine-wave signal having the desired phase and
amplitude can be synthesized, as shown in FIGS. 29-31. In the cases
of FIGS. 29-31, the two sine-wave signals having a phase difference
of 90.degree. are used to synthesize the composite sine-wave signal
by utilizing the same function table, as described below. However,
the two sine-wave signals to be used need not have a phase
difference of 90.degree., and may have any other desired phase
difference. For example, two sine-wave signals A1 and A2 having a
phase difference smaller than 90.degree., as indicated in FIG. 32,
may be combined to synthesize a composite sine-wave signal having
the desired phase and amplitude.
[0187] The function table 140 shown in FIG. 27 stores a function
table in the form of a sine-wave table indicating sampling values
corresponding to respective different phases at predetermined
sampling points. In the present fifth embodiment, the
digital-transmission-signal output portion 120 and the 90.degree.
phase shifting portion 141 are arranged to generate the
above-described first and second sine-wave signals, respectively,
on the basis of the function table stored in the function table
140. FIGS. 33-36 show examples of the sine-wave table (IF table)
stored in the function table 140.
[0188] In the examples of the sine-wave table shown in FIGS. 33-36,
the oscillation frequency is made equal to 1/4 of the sampling
frequency, for simplifying the control to generate the sine-wave
signals.
[0189] In the example of FIG. 33, different values cos .phi. are
stored in relation to respective different initial phase values
.phi.. Namely, the sine-wave table indicates successive discrete
values "1", "0", "-1" and "0" in relation to respective different
initial phase values .phi.=0.pi., .pi./2, 1.pi., and 3.pi./2. These
successive discrete values are read out from the
digital-transmission-signal output portion 120 and applied to the
first transmission-signal D/A converting portion 142, so that the
first sine-wave signal (cosine-wave signal) is generated by the
first transmission-signal D/A converting portion 142. The
90.degree. phase shifting portion 141 which has received the output
of the first transmission-signal output portion 120 delays the
moment of application of this output to the second
transmission-signal output portion 143, by a time corresponding to
one sampling value, with respect to the moment of application to
the first transmission-signal output portion 142, so that the
second transmission-signal D/A converting portion 143 generates the
second sine-wave signal the phase of which is different by
90.degree. from that of the first sine-wave signal. Alternatively,
the 90.degree. phase shifting portion 141 shifts by one position
the positions of the sine-wave table from which the successive
discrete values are read out in relation to the initial phase
values, and applies the thus read-out successive discrete values to
the second transmission-signal D/A converting portion 143 for
generating the second sine-wave signal. For instance, the
successive values "0", "1", "0" and "-1" are read out in relation
to the respective different initial phase values .phi.=0.pi.,
.pi./2, 1.pi., and 3.pi./2.
[0190] In the case of FIG. 34, a set of values cos .phi. for the
first sine-wave signal and a set of values sin .phi. for the second
sine-wave signal are stored in the function table 140. The first
transmission-signal output portion 120 reads out successive
discrete values "1", "0", "-1" and "0" in relation to the
respective initial phase values .phi.=0.pi., .pi./2, 1.pi., and
3.pi./2, so that the first transmission-signal D/A converting
portion 142 generates the first sine-wave signal, while the
90.degree. phase shifting portion 141 reads out successive discrete
values "0", "1", "0", and "-1" in relation to the respective
initial phase values .phi.=0.pi., .pi./2, 1.pi., and 3.pi./2, so
that the second transmission-signal D/A converting portion 143
generates the second sine-wave signal.
[0191] The sine-wave tables of FIGS. 33 and 34 may be replaced by
respective sine-wave tables of FIGS. 35 and 36. The sine-wave table
of FIG. 35 stores successive discrete values cos .phi. (e.g.,
"0.7071", "-0.7071", "-0.7071" and "0.7071") in relation to
respective different initial phases .phi.=.pi./4, 3.pi./4, 5.pi./4,
and 7.pi./4, while the sine-wave table of FIG. 36 stores successive
discrete values cos .phi. (e.g., "0.7071", "-0.7071", "-0.7071" and
"0.7071") and successive discrete values sin .phi. (e.g., "0.7071",
"0.7071", "-0.7071" and "-0.7071"), in relation to the respective
different initial phases .phi.=.pi./4, 3.pi./4, 5.pi./4, and
7.pi./4.
[0192] The first and second sine-wave signals having the phase
difference of 90 are generated by the first transmission-signal D/A
converting portion 142 and the second transmission-signal D/A
converting portion 143, on the basis of the read-out successive
discrete values shown in FIGS. 33-36 by way of example, and the
generated first and second sine-wave signals are applied to the
first-composite-sine-wave-signal generating circuit 180 and the
second-composite-sine-wave-signal generating circuit 190,
respectively.
[0193] As described above, the phase of the sine-wave signal to be
generated can be changed by changing the positions of the sine-wave
table in the function table 140 from which the successive discrete
values are read out, and the amplitude of the sine-wave signal can
be changed by multiplying the read-out sine-wave signal by a
control value.
[0194] Referring to FIG. 37, there is shown in detail an
arrangement of the first-composite-sine-wave-signal generating
circuit 180, which includes an amplifier 181, an amplifier 182, a
switch 183, a switch 184, a variable attenuator 185, a variable
attenuator 186, and an adder 187. The variable attenuator 185
operates according to the amplitude control signal Vatc1 (control
voltage) generated by the first-cancel-control-signal D/A
converting portion 153, as an amplitude control portion operable to
control the amplitude of the first sine-wave signal (cos .omega.t)
generated by the first transmission-signal D/A converting portion
142. The amplifier 181 has a gain of -1 for reversing the polarity
of the first sine-wave signal. The switch 183 operates according to
the polarity control signal Vswc1 generated by the
first-cancel-control signal output portion 124, to switch or
control the polarity of the first sine-wave signal. The variable
attenuator 186 operates according to the amplitude control signal
Vats1 (control voltage) generated by the
first-cancel-control-signal D/A converting portion 154, as an
amplitude control portion operable to control the amplitude of the
second sine-wave signal (sin .omega.t) generated by the second
transmission-signal D/A converting portion 143. The amplifier 182
has a gain of -1 for reversing the polarity of the second sine-wave
signal. The switch 184 operates according to the polarity control
signal Vsws1 generated by the second-cancel-control signal output
portion 124, to switch or control the polarity of the second
sine-wave signal. The adder 187 functions as a sine-wave
synthesizing portion operable to combine together the outputs of
the variable attenuators 185, 186, to synthesize the composite
sine-wave signal.
[0195] In the first-composite-sine-wave-signal generating circuit
180 arranged as described above, the amplitudes of the first and
second sine-wave signals are controlled by the respective variable
attenuators 185, 186 according to the respective amplitude control
signals (control voltages) Vatc1 and Vats1, and the polarities of
the first and second sine-wave signals are controlled according to
the respective polarity control signals Vswc1 and Vsws1, by the
amplifiers 181, 182 and switches 183, 184 which cooperate to
function as a polarity switching portion operable to change the
polarities of the first and second sine-wave signals. Accordingly,
the amplitudes and the polarities of the first sine-wave signal sin
.omega.t and the second sine-wave signal cos .omega.t that are to
be combined together by the adder 187 to synthesize the composite
sine-wave signal having the desired phase can be selected as
desired with a high degree of freedom. The thus synthesized
composite sine-wave signal is applied to the second up-converter
156.
[0196] The second-composite-sine-wave-signal generating circuit 190
has the same arrangement as the first-composite-sine-wave-signal
generating circuit 180 described above. In the
second-composite-sine-wave-signal generating circuit 190, the
amplitudes of the first and second sine-wave signals are controlled
by the respective variable attenuators 185, 186 according to the
respective amplitude control signals (control voltages) Vatc2 and
Vats2, and the polarities of the first and second sine-wave signals
are controlled according to the respective polarity control signals
Vswc2 and Vsws2, by the amplifiers 181, 182 and switches 183, 184
which cooperate to function as a polarity switching portion.
Accordingly, the amplitudes and the polarities of the first
sine-wave signal sin .omega.t and the second sine-wave signal cos
.omega.t that are to be combined together by the adder 187 to
synthesize the composite sine-wave signal having the desired phase
can be selected as desired with a high degree of freedom. The thus
synthesized composite sine-wave signal is applied to the second
signal combining portion 166. The first-composite-sine-wave-signal
generating circuit 180 and the second-composite-sine-wave-signal
generating circuit 180 are limited to the arrangement described
above, but may be otherwise arranged.
[0197] Referring to the circuit diagram of FIG. 38, there is shown
a modified first-composite-sine-wave-signal generating circuit 180'
provided in place of the first-composite-sine-wave signal
generating circuit 180 shown in FIG. 37. In this
composite-sine-wave-signal generating circuit 180', variable-gain
amplifiers 185', 186' are provided as amplitude control portions,
in place of the variable attenuators 185, 186 of FIG. 37. Like the
variable attenuator 185, the variable-gain amplifier 185' operates
according to the amplitude control signal (control voltage) Vatc1
generated by the first-cancel-control-signal D/A converting portion
153, to control the amplitude of the first sine-wave signal (cos
.omega.t) generated by the first transmission-signal D/A converting
portion 142. Like the variable attenuator 186, the variable:gain
amplifier 186' operates according to the polarity amplitude control
signal (control voltage) Vats1 generated by the
second-cancel-control-signal D/A converting portion 154, to control
the amplitude of the second sine-wave signal (sin .omega.t)
generated by the second transmission-signal D/A converting portion
143. In the other aspects, the arrangement and operation of the
first-composite-sine-wave-signal generating circuit 180' are
identical with those of the first-composite-sine-wave-signal
generating circuit 180. The second-composite-sine-wave-signal
generating circuit 190 of FIG. 37 may be replaced by a modified
second-composite-sine-wave-signal generating circuit 190' having
the same arrangement as the modified
first-composite-sine-wave-signal generating circuit 180'.
[0198] Referring to next to the circuit diagram of FIG. 39, there
is shown another modified first-composite-sine-wave-signal
generating circuit 180'' provided in place of the
first-composite-sine-wave signal generating circuit 180 of FIG. 37.
In FIG. 39, the same reference signs as used in FIGS. 37 and 38 are
used to identify the same elements. The description of these
elements is simplified or omitted.
[0199] The modified first-composite-sine-wave generating circuit
180'' includes an absolute-value circuit AVC1 for the first
sine-wave signal, and an absolute-value circuit AVC2 for the second
sine-wave signal. Each of the absolute-value circuits AVC1, AVC2
includes two converters C, two diodes D and five resistors R, as
known in the art.
[0200] The absolute-value circuit AVC1 is arranged to receive a
control voltage signal Vatc' (having a polarity as well as an
amplitude) corresponding to the amplitude control signals Vatc
(Vatc1 and Vatc2) described above, and applies an absolute value
|Vatc'| of the received control voltage signal Vatc' to
the-variable attenuator 185. The generating circuit 180'' includes
a comparator 188 having two inputs one of which is grounded. The
control voltage signal Vatc' is applied to the other input of the
comparator 188, which applies to the switch 183 a polarity control
signal indicative of the polarity (plus or minus) of the control
voltage signal Vatc', so that the switch 183 cooperates with the
amplifier 181 to control polarity of the first sine-wave
signal.
[0201] The absolute-value circuit AVC2 is arranged to receive a
control voltage signal Vats' (having a polarity as well as an
amplitude) corresponding to the amplitude control signals Vats
(Vats1 and Vats2) described above, and applies an absolute value
|Vats'| of the received control voltage signal Vats' to the
variable attenuator 186. The generating circuit 180'' includes a
comparator 189 having two inputs one of which is grounded. The
control voltage signal Vats' is applied to the other input of the
comparator 189, which applies to the switch 184 a polarity control
signal indicative of the polarity (plus or minus) of the control
voltage signal Vats', so that the switch 184 cooperates with the
amplifier 182 to control the polarity of the second sine-wave
signal.
[0202] The modified first-composite-sine-wave-signal generating
circuit 180'' does not require the polarity control signals Vswc1,
Vsws1 generated by the first-cancel-control-signal output portion
124, and the absolute-value circuits AVC1, AVC2 receive from the
D/A converting portions 153, 154 the above-described control
voltage signals Vatc1', Vats1' (having also the polarity), the
absolute values |Vatc1'| and |Vats1'| of which are applied to the
variable attenuators 185, 186, to control the amplitudes of the
first and second sine-wave signals. The polarity reversing portion
in the form of the amplifiers 181, 182 and switches 183, 184
controls the polarity of the first and second sine-wave signals,
according to the polarity control signals received from the
comparators 188, 189. This arrangement permits a high degree of
freedom to select the amplitudes and polarities of the first and
second sine-wave signals (sin .omega.t and cos .omega.t) that are
to be combined together by the adder 187 to synthesize the
composite sine-wave signal having the desired phase, which is to be
applied to the second up-converter 156.
[0203] The second-composite-sine-wave-signal generating circuit 190
of FIG. 37 may be replaced by a modified
second-composite-sine-wave-signal generating circuit 190'' having
the same arrangement as the modified
first-composite-sine-wave-signal generating circuit 180''. The
modified second-composite-sine-wave-signal generating circuit 190''
does not require the polarity control signals Vswc2, Vswc2
generated by the second-cancel-control-signal output portion 128,
and the absolute-value circuits AVC1, AVC2 receive from the D/A
converting portions 163, 164 the above-described control voltage
signals Vatc2', Vats2' (having also the polarity), the absolute
values |Vatc2'| and, |Vats2'| of which are applied to the variable
attenuators 185, 186, to control the amplitudes of the first and
second sine-wave signals. The polarity reversing portion in the
form of the amplifiers 181, 182 and switches 183, 184 controls the
polarity of the first and second sine-wave signals, according to
the polarity control signals received from the comparators 188,
189. This arrangement permits a high degree of freedom to select
the amplitudes and polarities of the first and second sine-wave
signals (sin .omega.t and cos .omega.t) that are to be combined
together to synthesize the composite sine-wave signal having the
desired phase, which is to be applied to the second signal
combining portion 166.
[0204] The modified composite-sine-wave-signal generating circuits
180'', 190'' can be simplified with a reduced number of signal
lines, since the generating circuits 180'', 190'' do not require
the amplitude control signals Vswc1, Vsws1 generated by the
first-cancel-control-signal output portion 125 or the polarity
control signals Vswc2, Vsws2 generated by the
second-cancel-control-signal output portion 128, as described
above.
[0205] It will be understood from the foregoing description that
the first transmission-signal D/A converting portion 142 and the
second transmission-signal D/A converting portion 143 cooperate to
constitute a sine-wave generating portion, and also a carrier-wave
output portion operable to generate the first sine-wave signal as a
carrier wave to gain an access to a desired object, while the
antenna 2 constitutes a transmitting portion operable to transmit
the carrier wave generated by the carrier-wave output portion, to
the desired object, and also a receiving portion operable to
receive a reply signal transmitted from the desired object in
response to the carrier wave. It will also be understood that the
first signal combining portion 158 constitutes a signal combining
portion operable to combine together the reply signal received by
the receiving portion and the cancel signal generated by a
sine-wave synthesizing portion, to obtain a first composite signal,
while the second-composite-signal A/D converting portion 172
constitutes an analog-to-digital converter operable to convert the
first composite signal into a digital signal.
[0206] It will also be understood that the first
transmission-signal D/A converting portion 142 constitutes a first
digital-to-analog converter operable to convert a set of sine-wave
sampling values into a first sine-wave signal, while the second
transmission-signal D/A converting portion 143 constitutes a second
digital-to-analog converter operable to convert a set of sine-wave
sampling values into a second sine-wave signal. It will further be
understood that the second up-converter 156 constitutes an
up-converting portion operable to increase the frequency of the
composite sine-wave signal generated by the sine-wave synthesizing
portion, while the first down-converter 162 constitutes a
down-converting portion operable to reduce the frequency of the
first composite signal generated by the signal combining
portion.
[0207] In the RFID-tag communication system S according to the
fifth embodiment of this invention described above, the first and
second sine-wave signals which are generated by the first and first
and second transmission-signal D/A converting portions 142, 143 and
which have a phase difference of 90.degree. are combined together
by the composite-sine-wave-signal generating circuits 180, 190 to
synthesize the composite sine-wave signals after the amplitudes of
the first and second sine-wave signals are controlled. The present
RFID-tag communication system S is capable of generating the
desired sine-wave signals, by combining together the sine-wave and
cosine-wave signals having the respective different amplitudes.
Thus, the present RFID-tag communication system S has a simple
arrangement for changing the phases of the composite sine-wave
signals as desired. Further, unlike an ordinary phase shifter, the
present arrangement does not suffer from a limited range of change
of the phase of the composite sine-wave signals.
[0208] Further, the clock signal applied to the first
transmission-signal D/A converting portion 142 and the clock signal
applied to the second-composite-signal A/D converting portion 172
are both generated by the same clock-signal output portion 178.
Namely, the same clock signal is applied to the D/A converting
portion 142 and the A/D converting portion 172. Accordingly, the
first transmission-signal D/A converting portion 142 arranged to
convert a set of sine-wave sampling values into the first sine-wave
signal, and the second-composite-signal A/D converting portion 172
arranged to convert the second composite signal (subjected to the
primary and secondary suppressions of the leakage signal) into a
digital signal can be operated in synchronization with each other.
This arrangement assures a high degree of demodulation of the
received signal, without a deviation of the frequency of the
received signal from the frequency of the transmission signal.
[0209] In addition, the provision of the second up-converter 156 to
increase the frequency of the composite sine-wave signal permits
the composite-sine-wave-signal generating circuit 180, 190 to
synthesize the sine-wave signals having a comparatively low
frequency, whereby the cost of manufacture of the communication
system S can be lowered. Further, the provision of the first
down-converter 162 to reduce the frequency of the first composite
signal generated by the first signal combining portion 158 permits
the second-composite-signal A/D converting portion 172 to convert
the first composite signal into the digital signal after the
frequency of the first composite signal has been reduced. In this
respect, too, the cost of manufacture of the communication system S
can be lowered.
Embodiment 6
[0210] Referring next to the functional block diagram of FIG. 40
corresponding to that of FIG. 27, there is shown a functional
arrangement of an interrogator which has an array antenna and which
is constructed according to the sixth embodiment of this invention.
In FIG. 40, the same reference signs as used in FIG. 27 are used to
identify the same elements, which will be only briefly described or
will not be described.
[0211] While the interrogator according to the preceding fifth
embodiment uses one transmitter/receiver antenna 2, the
interrogator according to the present sixth embodiment uses a
plurality of antennas, namely, three transmitter/receiver antennas
2-1, 2-2 and 2-3. The present interrogator includes a DSP 3'
incorporating three cancel processing portions 301-1, 301-2 and
301-3 which correspond to the respective three antennas 2-1, 2-2
and 2-3. Since the three cancel processing portions 301-1, 301-2
and 301-3 have the same circuit arrangement, the cancel processing
portion 301-1 corresponding to the antenna 2-1 will be described,
and the description of the cancel processing portion 301-1 applies
to the cancel processing portions 301-2 and 301-3. The elements
associated with the antennas 2-1, 2-2 and 2-3 are identified by
numerals 1, 2 and 3 following the reference signs used in the fifth
embodiment, and the description of these elements are simplified or
omitted.
[0212] Like the interrogator according to the preceding fifth
embodiment, the interrogator according to the present sixth
embodiment includes the function table 140,
digital-transmission-signal output portion 120, 90.degree. phase
shifting portion, modulating portion 122, first transmission-signal
D/A converting portion 142, second transmission-signal D/A
converting portion 143 and local-oscillation-signal output portion
144. The present interrogator further includes a phased-array (PAA)
processing portion 302 and an adaptive-array (AAA) processing
portion 303, which are characteristic of the present sixth
embodiment. The adaptive-array processing portion 303 is arranged
to control the directivity of the antennas 2-1, 2-2 and 2-3, in a
known manner on the basis of the reply signals received by those
three antennas 2-1, 2-2, 2-3, so as to maximize the sensitivity of
reception of the reply signals from the desired objects (RFID tags
T), or so as to minimize a ratio of failure of reception of the
reply signals. The phased-array processing portion 302 is arranged
to control the directivity of the antennas 2-1, 2-2 and 2-3 upon
transmission of the transmission signal toward the desired objects
(RFID tags T), such that the direction in which the antennas 2-1,
2-2, 2-3 have the highest gain and which is temporarily held is
sequentially changed, as described below in detail. As well known
in the art, the direction in which the antennas 2-1, 2-2 and 2-3
have the highest gain is determined by the spacing distance of the
three antennas, and the phase difference of the transmission
signals applied to those antennas. Therefore, it is necessary to
change the phase of the transmission signal to be applied to each
antenna 2, for changing the direction of the highest gain. The
elements of the DSP 3' other than the cancel processing portion
301-1, which are associated with the transmitter/receiver antenna
2-1, includes a first-cancel-control-signal output portion 124-1, a
second-cancel-control-signal output portion 128-1, and a
phased-array-control-signal output portion 305-1 operable to
generate a control signal for temporarily hold and sequentially
change the direction in which the antennas 2-1, 2-2 and 2-3 have
the highest gain during the transmission of the transmission signal
toward the RFID tags T. The cancel processing portion 301-1
includes a first-cancel-control-signal control portion 126-1, a
second-cancel-control-signal control portion 130-1, a demodulating
portion 134-1, a direct-current-component detecting portion 134-1,
a received-signal-amplitude detecting portion 136-1 and a
first-composite-signal-amplitude detecting portion 138-1.
[0213] In the present sixth embodiment, the digital transmission
signal generated by the digital-transmission-signal output portion
120 on the basis of the function table stored in the function table
140 is converted by the high-speed D/A converting portion 142 into
an analog signal (first sine-wave signal), as in the preceding
fifth embodiment. The transmission signal is generated by the
90.degree. phase shifting portion 141 on the basis of the signal
generated by the digital-transmission-signal output portion 120,
and this transmission signal is converted by the high-speed
transmission-signal D/A converting portion 143 into an analog
signal (second sine-wave signal), as in the fifth embodiment. The
first and second sine-wave signals generated by the
transmission-signal D/A converting portions 142 and 143 are applied
to the first-composite-sine-wave-signal generating circuit 180 and
the second-composite-sine-wave-signal generating circuit 190, and
also to a third-composite-sine-wave-signal generating circuit 304-1
(which will be described in detail).
[0214] The third-composite-sine-wave-signal generating circuit
304-1 is provided to control the amplitudes of the first and second
sine-wave signals and combine together these sine-wave-signals, on
the basis of a phased-array control signal generated by the
phased-array processing portion 302, to synthesize a composite
sine-wave signal the phase of which has been controlled with
respect to the antennas 2-1, 2-2, 2-3. The frequency of the
composite sine-wave signal generated by the
third-composite-sine-wave-signal generating circuit 304-1 is
increased by the first up-converter 146-1 by an amount
corresponding to the frequency of the local oscillation signal
generated by the local-oscillation-signal output portion 144, and
the amplitude of the same composite sine-wave signal is increased
by the first amplifying portion 148-1. Further, the composite
sine-wave signal in question is modulated according to a modulating
signal generated by the modulating portion 122. The transmission
signal generated by the first amplifying portion 148-1 is applied
to the antenna 2-1 through the transmission/reception separator
150-1, and is transmitted as an interrogating wave F, from the
antenna 2-1 toward the RFID-circuit element T.sub.0.
[0215] The reflected wave F transmitted from the RFID-circuit
element T.sub.0 and received by the antenna 2-1 is received as a
received signal by the first signal combining portion 158-1. The
present interrogator includes the second down-converter 174-1,
received-signal A/D converting portion 176-1,
received-signal-amplitude detecting portion 136 and
first-composite-signal-amplitude detecting portion 138 (which are
not shown), as in the fifth embodiment. The primary suppression or
cancellation (offsetting) of the leakage signal is effected by the
first signal combining portion 158-1. That is, the
first-cancel-control-signal output portion 126-1 determines the
phase and amplitude of the first cancel control signal on the basis
of the amplitude detected by the received-signal-amplitude
detecting portion 136-1, and the amplitude detected by the
first-composite-signal-amplitude detecting portion 138-1. The
determined phase and amplitude are applied to the
first-cancel-control-signal output portion 124-1, which generates
the first cancel control signal.
[0216] The first-composite-sine-wave-signal generating circuit
180-1 is provided to combine together the received first and second
sine-wave signals to synthesize the composite sine-wave signal
(first cancel signal for the primary suppression or cancellation of
the leakage signal), on the basis of the first cancel control
signal received from the first-cancel-control-signal output portion
124-1 (and the signals received through the low-speed
first-cancel-control-signal D/A converting portions 153-1, and
154-1), while the amplitudes of the first and second sine-wave
signals are controlled. The amplitude of the first cancel signal
generated by the first-composite-sine-wave-signal generating
circuit 180-1 is increased by the second up-converter 156-1, by an
amount corresponding to the frequency of the local oscillation
signal generated by the local-oscillation-signal output portion
144. The first cancel signal generated by the second up-converter
156-1 and the received signal received through the
transmission/reception separator 150-1 are combined together by the
first signal combining portion 158-1 to obtain the first composite
signal from which the leakage signal received by the interrogator
is totally or partially suppressed.
[0217] The amplitude of the first composite signal generated by the
first signal combining portion 158-1 is amplified by the second
amplifying portion 160-1, by a predetermined gain. Then, the
frequency of the first composite signal is reduced by the first
down-converter 162-1 by the amount corresponding to the frequency
of the local oscillation signal generated by the local-signal
output portion 144, and the first composite signal the frequency of
which has been reduced is applied to the second signal combining
portion 166-1 and the third amplifying portion 168-1 not shown. The
amplitude of the first composite signal applied to the third
amplifying portion 168-1 is amplified, and the amplified first
composite signal is converted into a digital signal by the
first-composite-signal A/D converting portion 170-1. The digital
first composite signal is applied to the
first-composite-signal-amplitude detecting portion 138-1, so that
the amplitude of the signal is detected. The output of the
first-composite-signal-amplitude detecting portion 138-1 is applied
to the first-cancel-control-signal control portion 126-1 and the
second-cancel-control-signal control portion 130-1.
[0218] The second-cancel-control-signal control portion 130-1
determines the phase and amplitude of the second cancel control
signal, on the basis of the output of the
first-composite-signal-amplitude detecting portion 138-1
(indicative of the amplitude of the received signal subjected to
the primary suppression of the leakage signal) and the second
composite signal generated by the second-composite-signal A/D
converting portion 172. The second-cancel-control-signal output
portion 128-1 generates the second cancel control signal for
generation of the second cancel signal having the determined phase
and amplitude. The second-composite-sine-wave-signal generating
circuit 190-1 is arranged to control the amplitudes of the first
and second sine-wave signals and combine together those sine-wave
signals, to obtain the second cancel signal, on the basis of the
second cancel control signal received from the
second-cancel-control-signal output portion 128-1 (and the signals
received through the low-speed second-cancel-control-signal D/A
converting portions 163-1, and 164-1), while the amplitudes of the
first and second sine-wave signals are controlled.
[0219] The second cancel signal generated by the
second-composite-sine-wave-signal generating circuit 190-1 and the
first composite signal the frequency of which has been reduced by
the first down-converter 162-1 are combined together by the second
signal combining portion 166-1 to obtain the second composite
signal from which the leakage signal has been totally or partially
suppressed.
[0220] The second composite signal generated by the second signal
combining portion 166-1 is converted into the digital signal by the
second-composite-signal A/D converting portion 172-1, and the
digital second composite signal is applied to the modulating
portion 132-1 and the adaptive-array processing portion 303, so
that the second composite signal from which the leakage signal has
been totally or partially suppressed is subjected to an adaptive
processing operation by the adaptive-array processing portion 303,
to read the reply signal which is received from the RFID-circuit
element T.sub.0 with the maximum sensitivity and which has a high
signal-to-noise ratio (a low ratio of reception failure). The
second composite signal demodulated by the demodulating portion
132-1 is applied to the direct-current-component detecting portion
134-1, so that the direct current component of the demodulated
signal is detected by the direct-current-component detecting
portion 134-1. An output of the detecting portion 134-1 is received
by the second-cancel-control-signal control portion 130-1. Since
the reply signal received from the RFID-circuit element T.sub.0 is
read by the adaptive-array processing portion 303, the demodulating
portion 132-1 operates to demodulate the second composite signal,
only for the purpose of permitting the direct-current-component
detecting portion 134-1 to detect the direct current component to
be applied to the second-cancel-control-signal control portion
130-1.
[0221] Referring to FIG. 41, there is shown in detail an
arrangement of the third-composite-sine-wave-signal generating
circuit 304, which is similar in arrangement to the
first-composite-sine-wave-signal generating circuit 180 and the
second-composite-sine-wave-signal generating circuit 190. The
third-composite-sine-wave-signal generating circuit 304 includes
the above-described amplifiers 181, 182, switches 183, 184,
variable attenuators 185, 186, and adder 187. The variable
attenuator 185 operates according to the amplitude control signal
(control voltage) which is generated by a low-speed D/A converting
portion 306-1 arranged to convert the digital signal received from
the phased-array-control-signal output portion 305-1 into an analog
signal. The variable attenuator 185 controls the amplitude of the
first sine-wave signal (cos .omega.t) generated by the high-speed
first transmission-signal D/A converting portion 142. The amplifier
181 is arranged to reverse the polarity of the first sine-wave
signal. The switch 183 operates according to the polarity control
signal generated by the phased-array-control-signal output portion
305-1, to switch or control the polarity of the first sine-wave
signal. The variable attenuator 186 operates according to the
amplitude control signal (control voltage) generated by a low-speed
D/A converting portion 307-1 arranged to convert the digital signal
received from the phased-array-control-signal output portion 305-1.
The variable attenuator 186 controls the amplitude of the second
sine-wave signal (sin .omega.t) generated by the high-speed second
transmission-signal D/A converting portion 143. The amplifier 182
is arranged to reverse the polarity of the second sine-wave signal.
The switch 184 operates according to the polarity control signal
generated by the phased-array-control-signal output portion 305-1,
to switch or control the polarity of the second sine-wave signal.
The adder 187 operates to combine together the outputs of the
variable attenuators 185, 186, to synthesize the composite
sine-wave signal.
[0222] In the third-composite-sine-wave-signal generating circuit
304 arranged as described above, the amplitudes of the first and
second sine-wave signals are controlled by the respective variable
attenuators 185, 186 according to the respective amplitude control
signals (control voltages) received from the D/A converting
portions 306-1 and 307-1, and the polarities of the first and
second sine-wave signals are controlled according to the respective
polarity control signals received from the
phased-array-control-signal output portion 305-1, by the amplifiers
181, 182 and switches 183, 184. Accordingly, the amplitudes and the
polarities of the first sine-wave signal sin .omega.t and the
second sine-wave signal cos .omega.t that are to be combined
together by the adder 187 to synthesize the composite sine-wave
signal having the desired phase can be selected as desired with a
high degree of freedom. The thus synthesized composite sine-wave
signal is applied to the first up-converter 146-1. The arrangement
and operation of the elements associated with the other antennas
2-2 and 2-3 are the same as those of the elements associated with
the antenna 2-1 which have been described.
[0223] In the present sixth embodiment, the leakage signal is
totally or partially suppressed from the received signals received
through the antennas 2-1, 2-2 and 2-3, so that the ratio
communication with the RFID tags can be effected with a high degree
of sensitivity, as in the preceding fifth embodiment.
[0224] The sixth embodiment has the following further advantages.
That is, an interrogator using a digital oscillator and an array
antenna device including a plurality of antennas as in the
interrogator of the present sixth embodiment generally requires a
high-speed D/A converter for each of the antennas, namely, a
plurality of D/A converters, and is accordingly expensive. In the
present sixth embodiment, however, the first transmission-signal
D/A converting portion 142 and the second transmission-signal D/A
converting portion 143 are provided commonly for the three antennas
2-1, 2-2 and 2-3, that is, a common oscillator sine-wave generating
portion is provided to generate the first and second sine-wave
signals which are amplified and combined together by the three
first-composite-sine-wave-signal generating circuits 180-1, 180-2
and 180-3, three second-composite-sine-wave-signal generating
circuits 190-1, 190-2 and 190-3, and three third
composite-sine-wave-signal generating circuits 304-1, 304-2 and
304-3, which are provided for the respective three antennas 2-1,
2-2 and 2-3. Accordingly, the interrogator using the three antennas
2-1, 2-2, 2-4 according to the present sixth embodiment can be made
simple in the circuit arrangement and is available at an
accordingly low cost.
[0225] Described in detail, generation of sine-wave signals of 10.7
MHz, for example, requires expensive high-speed D/A converters
which are operated at 42.8 MHz, for example. Where the first and
second cancel signals in addition to the transmission signal are
generated using a function table, three sets of high-speed D/A
converters are required for each of a plurality of antennas of the
array antenna device, so that the interrogator requires a
considerably large number of high-speed D/A converters, and is
complicated in the circuit arrangement and accordingly expensive.
In the interrogator 1 of the present sixth embodiment, however,
only the first and second transmission-signal D/A converting
portions 142, 143 are provided by respective expensive D/A
converters capable of high-speed processing operations to generate
the first and second sine-wave signals, but the
first-cancel-control-signal D/A converting portions 153, 154,
second-cancel-control-signal D/A converting portions 163, 164, and
phased-array-control-signal D/A converting portions 306, 307 can be
provided by inexpensive low-speed D/A converters to generate the
transmission signal and the first and second cancel signals having
the controlled amplitudes and phases, by using analog amplitude
control signals, whereby the interrogator 1 is available at a
relatively low cost. That is, the interrogator 1 can be made simple
in construction and accordingly inexpensive, since the frequency
required to control the amplitudes and phases of the first and
second cancel signals is considerably lower than the frequency of
the transmission signal.
[0226] In the sixth embodiment, the adaptive-array processing
operation is performed independently of the phased-array processing
operation, only for the reception of the reply signals. However,
the adaptive-array processing operation may be performed by the
adaptive-array processing portion 303, for the transmission of the
transmission signal as well as for the reception of the reply
signals, by using the weights determined in the adaptive-array
processing for the reception. Described in detail, the interrogator
1 may be provided with an adaptive-array-control-signal output
portion similar to the phased-array-control-signal output portion
305, and a composite-sine-wave-signal output portion operable
according to a control signal generated by the
adaptive-array-control signal output portion, for generating a
composite sine-wave signal having an amplitude and a phase which
are controlled to maximize the gain of the antennas 2-1, 2-2, 2-3
upon transmission of the transmission signal toward the RFID tags
T, on the basis of the direction in which the antennas 2 have the
highest sensitivity of reception of the reply signals from the RFID
tags T.
[0227] The composite-sine-wave-signal generating circuits 180, 190,
180', 190', 180'', 190'', 304 are arranged to receive the plurality
of input control signals as shown in FIGS. 37-39 and 41, these
generating circuits are not limited to the details shown in these
figures, and may modified to reduce the number of the input control
signals.
[0228] Referring to the circuit diagram of FIG. 42, there is shown
in detail an example of an arrangement of a modified
composite-sine-wave-signal generating circuit 280, 290. In FIG. 42,
the same reference signs as used in FIG. 37 are used to identify
the same elements, the description of which is omitted or
simplified.
[0229] The composite-sine-wave-signal generating circuit 280 shown
in FIG. 42 includes low-speed D/A converters 250, 260, a logic
circuit 270, a shift register 271 and a register 272. The logic
circuit 270 is operable as an amplitude control portion to generate
an amplitude control signal, on the basis of an input serial
signal. The shift register 271 and register 272 are operable as a
registering portion to generate an amplitude control signal and a
polarity control signal for controlling the first and second
sine-wave signals, on the basis of a serial-data signal extracted
by the logic circuit 270. The low-speed D/A converters 250, 260 are
arranged to convert the amplitude control signal into an analog
signal.
[0230] In the composite-sine-wave-signal generating circuit 280
arranged as described above, control signal line information and
data to be applied to the D/A converters 250, 260 are supplied as
serial data from the first-cancel-control-signal output portion 124
to the logic circuit 270. The shift register 271 receives the
serial data signal from the logic circuit 270, and converts the
received serial data signal into a parallel data signal. The lock
signal received from the clock-signal output portion 178 is made
effective by a start bit inserted before the received serial data,
and is made ineffective upon expiration of a predetermined time
defined by a predetermined bit received by the shift register 271,
so that the parallel data in the shift register 271 are latched by
the register 272, and the values of the bits (at least a portion of
the parallel data in the shift register 271) are applied as the
above-described amplitude control signal to the variable
attenuators 185, 186 through the D/A converters 250, 260, and are
applied as the polarity control signal to the switches 183,
184.
[0231] As described above, the composite-sine-wave-signal
generating circuit 280 is arranged to control the amplitudes and
phases of the first and second sine-wave signals according to the
amplitude control signal and the polarity control signal generated
on the basis of the parallel signal obtained by conversion of the
output of the logic circuit 270. Accordingly, the first and second
sine-wave signals can be controlled by using a single signal line
extending from the first-cancel-control-signal output portion 124
(or the second-cancel-control-signal output portion 128 or
phased-array-control-signal output portion 305).
[0232] The circuit diagram of FIG. 43 shows the interrogator of
FIG. 40 as modified to use the composite-sine-wave-signal
generating circuit 280 of FIG. 42 as each of the
first-composite-sine-wave-signal generating circuits 180-1, 180-2,
180-3, second composite-sine-wave-signal generating circuits 190-1,
190-2, 190-3, and third-composite-sine-wave-signal generating
circuits 304-1, 304-2, 304-3. As shown in FIG. 43, the number of
the signal lines required I the composite-sine-wave-signal
generating circuit 280 is effectively reduced, and the arrangement
of the generating circuit 280 is simplified.
Embodiment 7
[0233] Referring next to FIG. 44, there is shown an RFID-tag
communication device 400 constructed according to a seventh
embodiment of this invention, which includes a first-cancel-signal
attenuator 402, in addition to the elements provided in the
RFID-tag communication device 12 of FIG. 2. The first-cancel-signal
attenuator 402 is disposed to receive the analog first cancel
signal generated by the first-cancel-signal D/A converting portion
54, so that the analog first cancel signal is attenuated on the
basis of the control signal generated from the first-cancel-signal
control portion 26, to reduce its amplitude depending upon the
leakage signal that is a part of the transmission signal which is
transmitted from the RFID-tag communication device 400 and which is
returned to this communication device 400. The attenuated first
cancel signal is applied to the second up-converter 56. The
RFID-tag communication device 400 further includes a filter 404
interposed between the transmission/reception separator 50 and the
transmitter/receiver antenna 52, and a second-cancel-signal
attenuator 406 disposed to receive the analog second cancel signal
generated by the second-cancel-signal D/A converting portion 64, so
that the analog second cancel signal is attenuated on the basis of
the second control signal generated from the second-cancel-signal
control portion 30, to reduce its amplitude depending upon the
leakage signal. The attenuated second cancel signal is applied to
the second signal combining portion 66.
[0234] FIG. 45 shows an arrangement of the first-cancel-signal
attenuator 402. Since the second-cancel-signal attenuator 406 has
the same arrangement as the first-cancel-signal attenuator 402,
only the first-cancel-signal attenuator 402 will be described. As
shown in FIG. 45, the first-cancel-signal attenuator 402 includes a
switching device SW having resistors R1 and R2, five dividers 402a,
402b, 402c, 402d and 402e in the form of five resisters Ra, Rb, Rc,
Rd and Re, and five switches Sa, Sb, Sc, Sd and Se for selectively
operating the five dividers. The first-cancel-signal attenuator 402
further includes a buffer amplifier BA having a buffer function.
While the register R1 is grounded in the present embodiment, it may
be grounded through a suitable switch. The switching device SW is
preferably arranged to selectively connect the resisters Ra, Rc,
Rc, Rd, Re to the ground. Thus, the first-cancel-signal attenuator
402 is arranged to change the amount of attenuation of the cancel
signal to a selected one of different values. Preferably, the
different values to which the cancel signal can be selectively
attenuated are multiples of 1/2, namely, 1/2, 1/4, 1/8, 1/16 and
1/32, for example.
[0235] Referring to FIG. 46, there will be described a relationship
between the level of a leakage carrier wave and an amount of
suppression. The unnecessary or leakage signal mixed in the
received signal received by the communication device 400 includes a
leakage signal which is a part of the transmission signal (carrier
wave) which leaks from the transmission/reception separator 50, and
a leakage signal that is a part of the transmission signal which is
transmitted from the communication device 400 and which is
reflected by an external object and returned back to the
communication device 400. Where the communication device uses
separate transmitter antenna and receiver antenna, the leakage
signal mixed in the received signal includes a signal transmitted
by the transmitter antenna and received by the receiver antenna
directly or indirectly from the transmitter antenna. Where a 14-bit
D/A converter is used, the maximum amount of suppression is 50 dB,
as indicated in FIG. 46. When the amplitude of the leakage carrier
wave is reduced to 1/4, that is, when the leakage carrier wave
level is reduced to about 12 bits to about 10 bits (full-scale
input bit number equivalent of the D/A converter), the amount of
suppression is reduced by about 10 dB. When the leakage carrier
wave level is reduced to about 6 bits (full-scale input bit number
equivalent of the D/A converter), the amount of suppression is
reduced by about 15 dB. It is noted that a high-resolution D/A
converter has a relatively large conversion error, the phase error
increases with a decrease in the level of the generated signal, so
that the amount of suppression may be insufficient. In the present
RFID-tag communication device 400, the amount of attenuation of the
first cancel signal by the first-cancel-signal attenuator 402
connected to the output of the first-cancel-signal D/A converting
portion 54 is variable to a selected one of values 2.sup.-N
(namely, 1/2, 1/4, 1/8 . . . ). Accordingly, the amount of
attenuation can be changed as desired depending upon the amplitude
of the leakage signal. Thus, the amplitude of the first cancel
signal generated by the first-cancel-signal D/A converting portion
54 can be adjusted by the first-cancel-signal attenuator 402,
depending upon the level of the leakage carrier wave, so that the
output of the first-cancel-signal D/A converting portion 54 is held
between 2 and 3 bits (full-scale bit number equivalents).
Accordingly, the amount of suppression of about 45-51 dB is
obtained irrespectively of the level of the leakage carrier wave,
where the 14-bit D/A converter is used, so that the unnecessary or
leakage signal can be effectively suppressed from the received
signal. Fine adjustment of the amplitude and phase of the first
cancel signal can also be made according to the control signal
applied to the first-cancel-signal D/A converting portion 54. As
described above, four sampling values in the function table stored
in the function table 40 are used per one period, so that the phase
of the first cancel signal can be controlled with high accuracy in
a highly efficient manner. Further, the filter 404 interposed
between the transmission/reception separator 50 and the
transmitter/receiver antenna 52 can suppresse a quantization error
which is a higher harmonic, so that the transmission signal to be
transmitted from the communication device 400 has a high
signal-to-noise (SIN) ratio. Further, the filter 404 which prevents
an increase of the noise floor assures the high signal-to-noise
ratio in spite of the attenuation of the first cancel signal by the
first-cancel-signal attenuator 402.
[0236] Referring to the flow charts of FIGS. 47-50, there will be
described control operations of the DSP 16 of the RFID-tag
communication device 400 to suppresse the leakage signal. A control
routine shown in the flow charts corresponds to the control routine
shown in the flow charts of FIGS. 14-19. Described in detail, the
flow charts of FIGS. 47-40 respectively correspond to those of
FIGS. 14, 15, 17 and 18. The control operations of FIGS. 16 and 19
are also performed in the present seventh embodiment. The control
operations of the DSP 16 of the RFID-tag communication device 400
which are different from those in the first embodiment will be
described by reference to the flow charts of FIGS. 47-50.
[0237] The present control routine is initiated with step S101 of
FIG. 47 to reset the phase .phi.C1 of the first cancel signal and
.phi.C2 of the second cancel signal to "0". Then, the control flow
goes to step S102 to determine whether a command signal should be
transmitted toward the RFID tags 14. The transmission of the
command signal is requested in an upper-order control routine (not
shown). If an affirmative decision is obtained in step S102, the
control flow goes to step S103 corresponding to the modulating
portion 22 to modulate the above-described transmission signal
according to the command signal. Then, step S105 and the following
steps are implemented. If a negative decision is obtained in step
S102, the control flow goes to step S104 to inhibit the modulation
of the transmission signal, and then goes to the step S105 and the
flowing steps. The step S105 is provided to read the received
signal AD1 which has been converted into the digital signal by the
received-signal A/D converting portion 76. In the next step S106
corresponding to the received-signal-amplitude detecting portion
36, the amplitude of the received signal AD1 read in the step S105
is detected. In the next step S107, the amplitude A1 of the first
cancel signal is determined. Then, the control flow goes to step
S108 to reset the number "N" to "0", and to step S109 to determine
whether 2.sup.-(N+1) is smaller than A1/A1F. The value A1F is the
full-scale value. If a negative decision is obtained in the step
S108, the control flow goes to step S112 to increment the number
"N", and goes back to the step S109. If an affirmative decision is
obtained in the step S109, the control flow goes to step S110 to
set the amount of attenuation by the first-cancel-signal attenuator
402 to "2.sup.-N", and then to step S111 to set the value A1D to
"2NA1". Then, the control flow goes to step S113 and the following
steps of FIG. 48.
[0238] In the step S113 of FIG. 48, variables "i", "k" and "X" are
reset to "0". The control flow then goes to step S114 to read out
function values from the function table 40. In the next step S115,
the function values read out in the step S114 are multiplied by the
amplitude A1D determined in the step S111. The control flow then
goes to step S116 to read the first composite signal AD2 which has
been converted into the digital signal by the
first-composite-signal A/D converting portion 70. In the next step
S117, the amplitude AM1 of the first composite signal detected in
the step S116 is detected. Then, the control flow goes to step S118
to determine whether the amplitude AM1 of the first composite
signal detected in the step S117 is equal to or lower than a first
threshold value. If a negative decision is obtained in the step
S-118, the control flow goes to step S123 and the following steps.
If an affirmative decision is obtained in the step S118, the
control flow goes to step S119 to determine whether the gain G2 of
the third amplifying portion 68 is set at the maximum value. If an
affirmative decision is obtained in step S119, that is, if the gain
G2 of the third amplifying portion 68 is set at the maximum value,
the control flow goes to the step S21 of the flow chart of FIG. 16
described above with respect to the first embodiment. If a negative
decision is obtained in the step S119, that is, the gain G2 is not
set at the maximum value, the control flow goes to step S120 to
divide the variable. "X" by the gain G2 of the third amplifying
portion 68. The step S120 is followed by step S121 to add a
predetermined value dG to the gain G2, and step S122 to multiply
the variable "X" by the gain G2. Then, the control flow goes back
to the step S116. The step S123 is provided to determine whether
the amplitude AM1 of the first composite signal detected in the
step S117 is equal to or larger than a second threshold value. That
is, an optimum range of the amplitude AM1 in which the input
voltage of the first-composite-signal A/D converting portion 70 is
adequate is defined by the first threshold value that is a lower
limit, and a second threshold value that is an upper limit. If the
input voltage is initially lower than the first threshold value or
lower limit, the gain G2 of the third amplifying portion 68 should
be increased. If the affirmative decision is obtained in the step
S118, therefore, the control flow goes to the step S119 to
determine whether the gain G2 of the third amplifying portion 68 is
set at the maximum value. If the negative decision is obtained in
the step S118, the control flow goes to the step S123 to determine
whether the amplitude AM1 of the first composite signal detected in
the step S12 is equal to or higher than the second threshold value.
If a negative decision is obtained in the step S123, this means
that the input voltage of the first-composite-signal A/D converting
portion 70 is adequate. In this case, the control flow goes to step
S21 of the flow chart of FIG. 16 and the following steps. If an
affirmative decision is obtained in the step S123, that is, if the
input voltage of the first-composite-signal A/D converting portion
70 is higher than the second threshold value or upper limit, the
control flow goes to step S124 to determine whether the gain G2 is
set at the minimum value. If an affirmative decision is obtained in
the step S124, the control flow goes to the step S21 of the flow
chart FIG. 16 and the following steps. If a negative decision is
obtained in the step S124, the control flow goes to step S125 to
divide the variable "X" by the gain G2, and to step S126 to
subtract the predetermined value dG from the gain G2. Then, the
control flow goes to the step S122 and the following steps. Thus,
the gain G2 of the third amplifying portion 68 is controlled such
that the amplitude AM1 of the first composite signal (the input
voltage of the first-composite-signal A/D converting portion 70) is
held within the optimum range, so that the communication of the
RFID-tag communication device 400 with the RFID tags 14 can be
effected with high sensitivity, even when the amplitude of the
first composite signal generated by the first signal combining
portion 58 is reduced by the suppression of the leakage signal.
[0239] The control operation of the flow chart of FIG. 16 is
followed by step S136 of the flow chart of FIG. 49 in which the
predetermined value dG is added to the gain G1 of the second
amplifying portion 60. Then, step S137 is implemented to read the
first composite signal which has been converted into the digital
signal by the first-composite-signal A/D converting portion 70.
Step S138 is then implemented to detect the amplitude AM1 of the
first composite signal read in the step S138. The control flow then
goes to step S139 to determine whether the amplitude AM1 of the
first composite signal detected in the step S138 is set equal to a
predetermined value. If an affirmative decision is obtained in the
step S139, the control flow goes to step S141. If a negative
decision is obtained in the step S139, the control flow goes to
step S140 to determine whether the gain G1 of the second amplifying
portion 60 is set at the maximum value. If a negative decision is
obtained in the step S140, the control flow goes back to the step
S136 and the flowing steps. If an affirmative decision is obtained
in the step S140, the control flow goes to the above-indicated step
S141 to determine the amplitude A2 of the second cancel signal. The
step S141 is followed by step S142 to reset the number "N" to zero.
Then, step S143 is implemented to determine whether 2.sup.-(N+1) is
smaller than A2/A2F. The value A2F is the full-scale value. If a
negative decision is obtained in the step S143, the control flow
goes to step S146 to increment the number "N", and goes back to the
step S143. If an affirmative decision is obtained in the step S143,
the control flow goes to step S144 to set the amount of attenuation
by the second-cancel-signal attenuator 406 to "2.sup.-N". Then,
step S145 is implemented to set the value A2D to 2NA2. The control
flow then goes to the of FIG. 50 and the flowing steps. The
operation of the DSP 16 described above permits high-frequency
amplification by the second amplifying portion 60 of the received
signal the amplitude-modulated component of which is increased as a
result of suppression of the leakage signal. Accordingly, the
present RFID-tag communication device 400 permits detection of the
reply signals with high sensitivity, with a reduced influence of
the noise.
[0240] In the step S147 of FIG. 50, the variables "i", "k" and "Y"
are reset to "0". Then, step S148 is implemented to read out the
function values from the function table 40. The control flow then
goes to step S149 to multiply the function values read out in the
step S148, by the amplitude AD2 determined in the step S145. Step
S150 is then implemented to determine whether the demodulated
signal output of the demodulating portion 32 is present. If a
negative decision is obtained in the step S150, the control flow
goes to step S151 determine whether a predetermined time has
elapsed. If an affirmative decision is obtained in the step S151,
this means that the RFID tags 14 have not transmitted the reply
signals. In this case, the control flow goes back to the step S102
of FIG. 47 and the following steps. If a negative decision is
obtained in the step S151, the control flow goes back to the step
S150. If an affirmative decision is obtained in the step S150, that
is, if the demodulated signal output of the demodulating portion 32
is present, the control flow goes to step S152 corresponding to the
demodulating portion 32, to read the demodulated second composite
signal generated by the demodulating portion 32. Then, step S153
corresponding to the direct-current-component detecting portion 34
is implemented to detect the amplitude D2 of the direct current
component of the demodulated second composite signal. The control
flow then goes to step S154 to detect a maximum amplitude
AB.sub.max of the modulated second composite signal, and step S155
to determine whether the maximum amplitude AB.sub.max of the
demodulated signal detected in the step S154 is equal to or lower
than a first threshold value (which is different from the first
threshold value used in the step S118 of FIG. 48). That is, an
optimum range of the amplitude AB.sub.max in which the input
voltage of the second-composite-signal A/D converting portion 72 is
adequate is defined by the first threshold value that is a lower
limit, and a second threshold value that is an upper limit. If the
input voltage is initially lower than the first threshold value or
lower limit, the gain G3 of the second signal combining portion 66
should be increased. If an affirmative decision is obtained in the
step S155, therefore, the control flow goes to step S156 to
determine whether the gain G3 of the second signal combining
portion 66 is set at the maximum value. If a negative decision is
obtained in the step S155, the control flow goes to step S160 to
determine whether the amplitude AB.sub.max of the modulated second
composite signal detected in the step S154 is equal to or higher
than the second threshold value (which is different from the second
threshold value used in the step S123 of FIG. 48). If a negative
decision is obtained in the step S160', the control flow goes to
step S52 of FIG. 19 and the following steps. If an affirmative
decision is obtained in the step S160, that is, if the input
voltage of the second-composite-signal A/D converting portion 72 is
higher than the second threshold value or upper limit, the control
flow goes to step S161 to determine whether the gain G3 is set at
the minimum value. If an affirmative decision is obtained in the
step S156, that is, the gain G3 of the second signal combining
portion 66 is set at the maximum value, the control flow goes to
the step S52 of FIG. 19 and the following steps. If a negative
decision is obtained in the step S156, that is, if the gain G3 of
the second signal combining portion 66 is not set at the maximum
value, the control flow goes to step S157 to divide the variable
"Y" by the gain G3. The step S157 is followed by step S158 to add a
predetermined value dG to the gain G3, to increase the amplifying
factor of the second signal combining portion 66, and step S159 to
multiply the variable "Y" by the gain G3. Then, the control flow
goes back to the step S150. If an affirmative decision is obtained
in the step S161, that is, if the gain G3 of the second signal
combining portion 66 is set at the minimum value, even where the
input voltage of the second-composite-signal A/D converting portion
72 is higher than the upper limit, the control flow goes to the
step S52 of FIG. 16 and the following steps. If a negative decision
is obtained in the step S161, that is, if the gain G3 of the second
signal combining portion 66 is not set at the minimum value, the
control flow goes to step S162 to divide the variable "Y" by the
gain G3, and to step S163 to subtract the predetermined value dG
from the gain G3, to reduce the amplifying factor of the second
signal combining portion 66. Then, the control flow goes to the
step S159 and the following steps.
[0241] As described above, the RFID-tag communication device 440
includes: the first-cancel-signal output portion 24 operable to
generate the first cancel signal in the form of a digital signal
for suppressing from the received signal the leakage signal that is
a part of the transmission signal which is transmitted from the
transmitter/receiver antenna 52 and which is returned to and
received by the antenna 52; the first-cancel-signal D/A converting
portion 54 operable to convert the first cancel signal generated by
the first-cancel-signal output portion 24, into an analog signal;
the first-cancel-signal control portion 26 operable to generate a
first control signal for controlling the amplitude A1 and/or the
phase .phi.C1 of the first cancel signal generated by the
first-cancel-signal output portion 24; the first-cancel-signal
attenuator 402 operable according to the first control signal
generated by the first-cancel-signal control portion, to attenuate
the analog first cancel signal generated by the first-cancel-signal
D/A converting portion 54, to an amplitude corresponding to the
leakage signal; and the first signal combining portion 58 operable
to combine together the first cancel signal which has been
attenuated by the first-cancel-signal attenuator 402, and the
received signal, to obtain the first composite signal. Accordingly,
the present RFID-tag communication device 400 permits effective
suppression of the leakage signal by adequately controlling the
first-cancel-signal attenuator 402 depending upon the level of the
leakage signal, even where the leakage signal has a comparatively
low intensity. Namely, the present RFID-tag communication device 12
is capable of suppressing the leakage signal with a simple
arrangement, irrespective of the level or intensity of the leakage
signal.
[0242] The first-cancel-signal attenuator 402 may be arranged to
attenuate the amplitude A1 of the first cancel signal to a value
which is closest to but not larger than the amplitude of the
leakage signal. In this case, the amplitude of the first control
signal generated by the first-cancel-signal control portion 26 can
be made equal to the maximum output amplitude of the
first-cancel-signal D/A converting portion 54, or equal to a value
close to 1/2 of the maximum output amplitude, so that the amplitude
A1 and phase .phi.Cl of the first cancel signal can be accurately
controlled, and the leakage signal can be effectively
suppressed.
[0243] The first-cancel-signal attenuator 402 is arranged to
attenuate the amplitude A1 of the first cancel signal to a value
which is larger than and closest to the amplitude of the leakage
signal. In this case, the leakage signal can be effectively
suppressed.
[0244] Further, the first-cancel-signal control portion 26 is
arranged to generate the first control signal which causes the
amplitude of the first cancel signal generated by the
first-cancel-signal D/A converting portion 54, to be equal to 1/2
of the maximum amplitude, so that the leakage signal can be
sufficiently suppressed.
[0245] The RFID-tag communication device 440 further includes: the
second-cancel-signal output portion 28 operable to generate the
second cancel signal in the form of a digital signal for
suppressing from the received signal the leakage signal the
second-cancel-signal D/A converting portion 64 operable to convert
the second cancel signal generated by the second-cancel-signal
output portion 24, into an analog signal; the second-cancel-signal
output control 30 operable to generate a second control signal for
controlling the amplitude A2 and/or the phase .phi.C2 of the second
cancel signal generated by the second-cancel-signal output portion
28; the second-cancel-signal attenuator 406 operable according to
the second control signal generated by the second-cancel-signal
control portion 30, to attenuate the analog second cancel signal
generated by the second-cancel-signal D/A converting portion 64, to
an amplitude corresponding to the leakage signal; and the second
signal combining portion 66 operable to combine together the second
cancel signal which has been attenuated by the second-cancel-signal
attenuator 406, and the first composite signal generated by the
first signal combining portion 58, to obtain the second composite
signal. Accordingly, the present RFID-tag communication device 400
permits secondary suppression of the leakage signal at the second
signal combining portion 66, as well as primary suppression of the
leakage signal at the first signal combining portion 58, making it
possible to further improve the signal-to-noise ratio.
[0246] The first-cancel-signal attenuator 402 and the
second-cancel-signal attenuator 406 are arranged to change its
amount of attenuation of the first and second cancel signals, to a
selected one of different values, so that these attenuators 402,
406 can be simplified in construction, and the control to attenuate
the cancel signals can be simplified.
[0247] Further, the different values to which the amount of
attenuation of the first and second cancel signals by the
attenuators 402, 406 is variable are multiples of 1/2, so that the
circuit arrangement of the attenuators 402, 406 can be made
considerably simple. Further, the control to attenuate the cancel
signals can be simplified by a bit-shift logic using binary digits,
for example.
[0248] Each of the attenuators 402, 406 includes the plurality of
voltage dividers 402a through 402e in the form of the plurality of
registers Ra through Re, and the plurality of switches Sa through
Se operable to selectively operate the plurality of voltage
dividers 402a through 402e. Thus, the attenuators 402, 406 are
simple in construction and economical to manufacture.
[0249] The attenuator 402, 406 further includes the buffer
amplifier BA functioning as a buffer device, which assures a stable
operation of the attenuator.
[0250] Since the four sampling values are used by the cancel-signal
D/A converting portions 54, 64, per one period of the output
periodic function, the noise floor can be held low, permitting
high-speed converting operations of the converting portions, and
assuring a high signal-to-noise ratio of the analog cancel signals
which have been attenuated by the attenuators 402, 406.
Embodiment 8
[0251] Referring next to FIG. 51, there is shown an arrangement of
an RFID-tag communication device 408 constructed according to an
eighth embodiment of this invention, wherein the
first-cancel-signal attenuator 402 is interposed between the second
up-converter 56 and the first signal combining portion 58. The
first-cancel-signal attenuator 402 attenuates the amplitude of the
first cancel signal the frequency of which has been increased by
the second up-converter 56, to a value corresponding to the leakage
signal. Like the RFID-tag communication device 400 of the preceding
seventh embodiment, this present RFID0-tag communication device 408
permits effective suppression of the leakage signal.
Embodiment 9
[0252] Referring to FIG. 52, there is shown an arrangement of an
RFID-tag communication device 410 constructed according to a ninth
embodiment of this invention, which includes a transmission-signal
register 412 interposed between the modulating portion 22 of the
DSP 16 and the transmission-signal D/A converting portion 42 of the
transmitter/receiver circuit 18, and further includes a
first-cancel-signal register 414 interposed between the
first-cancel-signal output portion 24 of the DSP 16 and the
first-cancel-signal D/A converting portion 54 of the circuit 18.
The communication device 410 further includes: a high-speed
clock-signal output portion 416 arranged to generate a clock signal
to be applied to the transmission-signal D/A converting portion 42
and the first-cancel-signal D/A converting portion 54; a first
frequency divider 418 arranged to supply the clock signal generated
by the high-speed clock-signal output portion 416, to the
second-cancel-signal D/A converting portion 64,
first-composite-signal A/D converting portion 70, second
composite-signal A/D converting portion 72, and received-signal A/D
converting portion 76; a second frequency divider 420 arranged to
supply the clock signal generated by the high-speed clock-signal
output portion 416, to a PLL 422; and the PLL 422 arranged to
generate a local oscillation signal having a predetermined
frequency, according to the clock signal supplied from the second
frequency divider 420, and supply the generated local oscillation
signal to the first down-converter 62 and the second down-converter
74. It is possible to use 8-bit D/A converters for obtaining the
amount of suppression of at least 20 dB, as indicated in FIG. 46.
Since the 8-bit D/A converters are capable of comparatively
high-speed D/A converting operations, the present RFID-tag
communication device 410 is arranged to directly generate radio
frequency signals. The modulated signal and the first cancel signal
are temporarily stored in the registers 412, 414, which are
accessible at a high speed. The present communication device 410
permits high-speed processing operations while assuring a high
signal-to-noise ratio.
[0253] While the preferred embodiments of the present invention
have been described in detail by reference to the drawings, it is
to be understood that the invention is not limited to the details
of the illustrated embodiments, and may be otherwise embodied.
[0254] Although the communication devices 400, 408, 410 according
to the seventh through ninth embodiments includes both the
first-cancel-signal attenuator 402 corresponding to the first
cancel signal and the second-cancel-signal attenuator 406
corresponding to the second cancel signal, the communication device
may include only one of those two attenuators 402, 406, provided
the unnecessary or leakage signal can be sufficiently
suppressed.
[0255] It is to be understood that the present invention may be
embodied with various other changes not specifically described,
without departing from the spirit of the invention.
* * * * *